Add namespace plugin migration guide to docs.

[hreikin]

Hi, just a small documentation addition. This PR adds the blog post for migrating legacy plugins from the pelican blog to the documentation for better visibility.

Pull Request Checklist

I am not sure if the first three items in the list apply as this is only a documentation change ?

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

Thanks