Update user docs for Central v2024.3

[matthew-white]

• Offline entities (@lognaturel TODO)
  • Info about submissions that are held for processing Show events about offline entities backlog on submission details page central#821
• View deleted Submissions and delete/undelete Submission #1873 (@issa-tseng TODO)
• Upgrade instructions (@lognaturel TODO)
  • If you use forms that create or update entities, you'll also need to update Collect (because the entity spec version will get bumped) - Migrate forms to use offline entities central#692
• Entity properties with same name but different capitalization can't be created anymore

[lognaturel]

Entity properties with same name but different capitalization can't be created anymore

I don't feel like we need to document this. The error is very clear and I don't think most users would expect this to work.