Add Daily Digest and Extension Shortcuts to third-party extensions

[stdAri]

Adds two extensions by @stdAri to the community extensions list (README and repositories.json):

• Daily Digest: scheduled AI digests of new articles (combined or per category) via any OpenAI-compatible API, with numbered citations deep-linking into FreshRSS.
• Extension Shortcuts: lists every enabled extension that has a configuration page in the settings menu (sidebar and header gear menu), so its settings are one click away.

[Alkarex]

Please add to https://github.com/FreshRSS/Extensions/blob/main/repositories.json as well

[stdAri]

Done, added to repositories.json (and rebased on main). Thanks!

[stdAri]

I also added a second, small extension of mine (Extension Shortcuts) to this PR, since both go in the same "By @stdAri" section and separate PRs would conflict. Happy to split it into its own PR if you prefer.