docs: general cleanup and improvements

[ImRodry]

Please describe the changes this PR makes and why it should be merged:
I went through every page on the docs and added/fixed some things. Although the title says docs this does include very small changes to the typings and code which I will be detailing below and you're welcome to give your opinion on them if you believe they should be reverted.

⚠️ This PR contains the following breaking changes:

• Removed the RELAY_ENABLED guild feature as it was not found on the docs
• Marked Guild#preferredLocale as nullable since a Guild may not have community enabled
• Removed the MANAGED_EMOJI, GROUP_DM_CREATE and RPC_HAS_CONNECTED Application flags as they could not be found on the docs
• Changed the default format for the Emoji CDN endpoint to 'webp' to match the other endpoints and the docs
• Renamed the PINS_ADD message type to CHANNEL_PINNED_MESSAGE to match the Discord API docs

Along with those, here are the other changes:

• Added the Application Webhook type
• Added back the Collection @external link
• Fixed some wrong string types where they should be Snowflake
• Removed nested properties (e.g. options.x) and added them to separate typedefs
• Added monospace markdown to references to properties within typedefs
• Added mentioned class links in multiple descriptions
• Fixed a weird code example in the guild ban and kick methods
• Changed all "Promise<T>|Promise<T2>" types to "Promise<T|T2>"
• Removed redundant @returns descriptions
• Fixed typos
• Added links to Discord documentation in most enums whenever I was able to find them
• Moved a sentence in the Snowflake typedef out of the code block to prevent formatting issues (not sure if this will cause other issues, please let me know if it does)
• Created the DynamicImageFormat type and assigned it to the CDN endpoints that support dynamic images (including DynamicImageFormat which supported fewer types before)
• Renamed the ImageSize type to AllowedImageSize for consistency and assigned it to all CDN endpoint sizes (change previously suggested in feat(GuildMember): add guild avatars #5696 but might as well do it now).

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[iCrawl]

Apart from the changes requested 👍

[ImRodry]

Seems like the CI got stuck, can someone rerun it? It took long but we got there

[ImRodry]

The CI is just broken today apparently

[SpaceEEC]

There seems to be a partial outage, although the workflow is now at least queued.

[kyranet]

Besides the requested changes, LGTM

Stale

[Nytelife26]

Some textual errors in this PR, namely writing id instead of ID.

[ImRodry]

Some textual errors in this PR, namely writing id instead of ID.

Both are fine but if you wanted that changed you should’ve probably said that before merge

[DaStormer]

id is used throughout now anyway, refer to #6036