DOC-2367 remove create-a-skin.adoc from TinyMCE 6 Docs

[FarzadHayat]

Ticket: DOC-2367

• Creating a Skin page removed (used to be here http://docs-hotfix-6-doc-2367.staging.tiny.cloud/docs/tinymce/6/creating-a-skin/).
• Creating a Skin nav entry removed (used to be under http://docs-hotfix-6-doc-2367.staging.tiny.cloud/docs/tinymce/6/customize-ui/ in nav).

Changes:

• DOC-2367 remove create-a-skin.adoc from TinyMCE 6 Docs

Pre-checks:

• Branch prefixed with feature/6/ or hotfix/6/
• modules/ROOT/nav.adoc has been updated (if applicable)
• [-] Files has been included where required (if applicable)
• Files removed have been deleted, not just excluded from the build (if applicable)
• [-] Files added for New product features, and included a release note entry.

Review:

• Documentation Team Lead has reviewed

[kemister85]

LGTM