Document new OAuth changes for 4.3.0

[ThisIsMissEm]

• [x] Add /.well-known/oauth-authorization-server documentation, per https://github.com/mastodon/mastodon/pull/29191
• [x] Document deprecation of redirect_uri on Application and addition of redirect_uris
• [x] Document added support for mutliple redirect URIs for OAuth Applications, per https://github.com/mastodon/mastodon/pull/29192
• [x] Add warning hints around client_id, client_secret, access_token and code values that they should be treated as if they are password, and stored securely.
• [x] Document removal of required read scope for GET /api/v1/apps/verify_credentials (this now just requires a valid access token), per https://github.com/mastodon/mastodon/pull/27142
• [x] Reworked OAuth Scopes page to be clearer.
• [x] Document client_secret_expires_at on Application, per https://github.com/mastodon/mastodon/pull/30317/
• [x] Document the fact that on 4.2.x Applications could be deleted at any time and result in broken authentication flows, fixed by https://github.com/mastodon/mastodon/pull/30316
• [x] Document Application vs CredentialApplication split, per https://github.com/mastodon/mastodon/pull/29192
• [x] Document new profile scope, per https://github.com/mastodon/mastodon/pull/30357 & https://github.com/mastodon/mastodon/pull/29087
• [ ] Document PKCE Extension, and why using PKCE is recommended for all applications, per: https://github.com/mastodon/mastodon/pull/31129
• [ ] Document Removal of OAuth Resource Owner Password Grant Flow, per: https://github.com/mastodon/mastodon/pull/30960

This branch is based on #1444

[ThisIsMissEm]

I have noticed that there is some churn here due to my editor using Prettier for markdown documents. We may want to consider adopting prettier for this repository.

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[ThisIsMissEm]

Have address majority of the code review comments and left replies where I disagree with said comments or need more information.

[github-actions[bot]]

This pull request has resolved merge conflicts and is ready for review.

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[github-actions[bot]]

This pull request has resolved merge conflicts and is ready for review.

[renchap]

Do we need anything here before merging it, as the 4.3 beta has been released?

[ThisIsMissEm]

Do we need anything here before merging it, as the 4.3 beta has been released?

@renchap I can follow up with a new PR for the last two items

[ThisIsMissEm]

@renchap have rebased this and finished the two remaining tasks.

[oneiros]

I took the liberty to commit my two small adjustments, so that we can finally merge.

Feel free to open up a follow-up PR if you disagree with my changes. And thanks for all the hard work on this!

[ThisIsMissEm]

@oneiros those changes looked good, I think in the future reworking all this documentation to be more like AT Protocol's OAuth documentation might be a good idea. I think the current tutorial-based documentation leaves a lot to be desired.