Audit the wiki pages

Some of our wiki pages are outdated. User manual or developer guide type information should move into the sphinx docs for better visibility and so that it's versioned with the code. I think it makes sense to use the wiki for longer-term planning or planning of big changes, with the target audience being beets contributors. Where possible smaller plans can go to issues on beets with appropriate labels, and pages planning many separate changes can be broken up. That would minimise duplication of information and keep discussions together. GitHub milestones can also be used like epics to group sub-issues.

Here are some thoughts so we can discuss:

. | Wiki page | Type | Proposed change
--|------------- | -------|----------------------
:x: | [Architecture](https://github.com/beetbox/beets/wiki/Architecture) | Dev reference | Move to sphinx #3295
:heavy_check_mark: | [Attachment Proposal](https://github.com/beetbox/beets/wiki/Attachment-Proposal) | Planning | None
:heavy_check_mark: | [Attachments](https://github.com/beetbox/beets/wiki/Attachments) | Planning | None
:thought_balloon: | [Beets Web UI](https://github.com/beetbox/beets/wiki/Beets-Web-UI) | Planning | This is outdated. Move any unimplemented features to issues.
:thought_balloon: | [Concepts](https://github.com/beetbox/beets/wiki/Concepts) | User documentation | Move to sphinx #2277
:x: | [Cookbook](https://github.com/beetbox/beets/wiki/Cookbook) | User documentation | Move to sphinx #3285
:x: | [Database structure](https://github.com/beetbox/beets/wiki/Database-structure) | Dev reference | ~~Probably outdated. Delete or move to sphinx.~~ Deleted.
:thought_balloon: | [Hacking](https://github.com/beetbox/beets/wiki/Hacking) | Developer reference | Move to a new CONTRIBUTING file #2103
:heavy_check_mark: | [Issues](https://github.com/beetbox/beets/wiki/Issues) | Maintainer reference | ~~We can probably delete this?~~ Kept
:thought_balloon: | [MacOSX](https://github.com/beetbox/beets/wiki/MacOSX) | User documentation | Probably not relevant any more? If it is then move to sphinx.
:x: | [MediaFile](https://github.com/beetbox/beets/wiki/MediaFile) | Developer reference | ~~Delete after confirming migration to beetbox/mediafile sphinx docs.~~ Deleted
:heavy_check_mark: | [Optimization](https://github.com/beetbox/beets/wiki/Optimization) | Planning | None
:thought_balloon: | [Packaging](https://github.com/beetbox/beets/wiki/Packaging) | Packager reference | Delete but add a new page to the sphinx docs for distro packagers.
:thought_balloon: | [Playlists & CUE Sheets](https://github.com/beetbox/beets/wiki/Playlists-&-CUE-Sheets) | Planning | Merge into attachment proposal or standalone issues.
:heavy_check_mark: | [PluginAPI](https://github.com/beetbox/beets/wiki/PluginAPI) | Planning | None
:x: | [PriorArt](https://github.com/beetbox/beets/wiki/PriorArt) | Planning | ~~Not sure of the purpose. Maybe extract into concrete proposals?~~ Deleted
:heavy_check_mark: | [Python3](https://github.com/beetbox/beets/wiki/Python3) | Planning | None
:thought_balloon: | [Refactoring](https://github.com/beetbox/beets/wiki/Refactoring) | Planning | Break into separate issues with a refactor label
:heavy_check_mark: | [ReleaseChecklist](https://github.com/beetbox/beets/wiki/ReleaseChecklist) | Maintainer reference | ~~Maybe move to comments in `release.py`?~~ Kept
:thought_balloon: | [ReleasePlanning](https://github.com/beetbox/beets/wiki/ReleasePlanning) | Planning | Merge with other planning pages/issues
:thought_balloon: | [String literals](https://github.com/beetbox/beets/wiki/String-literals) | Developer reference | Also move to CONTRIBUTING file
:thought_balloon: | [Testing](https://github.com/beetbox/beets/wiki/Testing) | Developer reference | Also move to CONTRIBUTING file
:thought_balloon: | Meet the users/devs | Community | Proposed in #2386.