add a deprecations and removals guide (PennyLaneAI#5083)

Link to the doc:
https://xanaduai-pennylane--5083.com.readthedocs.build/en/5083/development/guide/deprecations_removals.html

**Context:**
There are a bunch of things to consider when deprecating code, so I
thought I'd consolidate my process in a document for posterity.

**Description of the Change:**
Adding a step-by-step guide to deprecations and removals to the public
developer docs.

**Benefits:**
PennyLane maintainers will know how we deprecate code forevermore

**Possible Drawbacks:**
what if this document itself gets out-of-date? 😨 

[sc-54330]

---------

Co-authored-by: Thomas R. Bromley <[email protected]>

doc/development/guide.rst

@@ -55,6 +55,11 @@ of PennyLane, plugins, and devices.
     :description: Architectural overview of PennyLane, its plugins and devices.
     :link: guide/architecture.html
 
+.. title-card::
+    :name: Deprecations and Removals
+    :description: Ensuring safety when introducing breaking changes to PennyLane.
+    :link: guide/deprecations_removals.html
+
 .. raw:: html
 
         <div style='clear:both'></div>
@@ -71,4 +76,5 @@ of PennyLane, plugins, and devices.
     guide/adr
     guide/architecture
     guide/logging
+    guide/deprecations_removals
 ..    guide/bestpractices

doc/development/guide/deprecations_removals.rst

@@ -0,0 +1,85 @@
+Deprecations and Removals
+=========================
+
+PennyLane is under continuous development and we sometimes need to make breaking changes to improve
+the library. When these breaking changes are necessary, we should make sure to give our users time
+to update their workflows to adhere to any new implementation before completely removing the old
+one. All ongoing and completed deprecations can be found in :doc:`the deprecations page <../deprecations>`.
+
+Deprecating a feature
+---------------------
+
+The means of informing users of a breaking change is called a deprecation cycle. Before removing a
+feature, for one or more releases of PennyLane, the deprecated feature should advise users of the
+upcoming change while preserving its functionality. Here are the steps needed to properly deprecate
+a feature:
+
+1. Identify the new or preferred way of achieving the same functionality once the deprecated
+   feature is removed. The exception to this is when a feature is being removed for lack of use.
+
+2. At the beginning of the relevant code (eg. the ``__init__()`` method of a class that's being
+   removed), add the following lines of code, filling in the relevant details:
+
+   .. code-block:: python
+
+       warnings.warn(
+           "<name-of-feature> is deprecated and will be removed in version <target-version>. "
+           "Instead, please use <name-of-preferred-way-to-achieve-functionality>.",
+           qml.PennyLaneDeprecationWarning,
+       )
+
+   If the feature is being relocated, consider rephrashing the warning to discuss relocation as
+   opposed to deprecation. As well, if the deprecated feature is fairly minor, it is likely safe to
+   state that it will be removed in the next release. Otherwise, it is a good practice to wait 2 or
+   more releases before fully removing the feature. Consult the product manager if you are not sure
+   how long this should be.
+
+3. If the feature has public-facing docs, include a similar warning message in a visible part of
+   its docstring using a ``.. warning::`` Sphinx directive.
+
+4. Replace all uses of the deprecated code within PennyLane's own source code. This is to ensure
+   that the warning isn't unexpectedly raised for users that did not personally call the deprecated
+   piece of code.
+
+5. Add a test to ensure that the deprecation warning is being raised. It should look similar to the
+   following code:
+
+   .. code-block:: python
+
+       def test_my_feature_is_deprecated():
+           """Test that my_feature is deprecated."""
+           with pytest.warns(qml.PennyLaneDeprecationWarning, match="my_feature is deprecated"):
+               _ = my_feature()
+
+6. Update any tests that specifically cover the deprecated feature to call it inside a
+   ``pytest.warns`` context as shown above. For tests that depend on it but are not written to
+   test it specifically, update them to use the new/preferred code.
+
+7. Add an entry to the top of the "Pending deprecations" section of ``doc/development/deprecations.rst``.
+   There should be existing examples to follow for style.
+
+8. Add a similar entry to the bottom of the "Deprecations" section in ``doc/releases/changelog-dev.md``.
+
+Additional notes on deprecations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- ``PennyLaneDeprecationWarning``'s are automatically raised as errors in PennyLane tests. If any
+  uses of the deprecated code are accidentally left in (tested) PennyLane code, CI will alert you
+  with a failure.
+- If a feature is flagged for deprecation but is still (significantly) used within PennyLane, it
+  is better to change that in a separate PR before deprecating the feature. You might find that the
+  feature should not be deprecated at all!
+
+Removing a deprecated feature
+-----------------------------
+
+Once a feature has been deprecated for a sufficiently long time, it is considered safe for removal.
+Here are the steps needed to properly remove a deprecated feature:
+
+1. Remove the deprecated source code, along with all tests that cover it.
+
+2. In ``doc/development/deprecations.rst``, move the existing deprecation entry to the "Completed
+   deprecation cycles" section below. Be sure to update the language to state that it has been
+   removed, and ensure that the PennyLane version in which it's being removed is correct.
+
+3. Add a similar entry to the bottom of the "Breaking Changes" section in ``doc/releases/changelog-dev.md``.