Update manual translation docs

[abdnh]

• Add more detailed instructions to translators/anki/manual and link to that page in https://github.com/ankitects/anki/tree/main/docs-site#translations instead of the other way around.
• Add a brief mention of docs translations to contributing.md.

[wofiporia]

Hi, I'd like to work on this.

Plan:

1. Expand docs-site/translators/anki/manual.mdx with more detailed instructions for translating the manual and other docs (forking the repo, which files/directories to edit, how to update docs.json, how to submit a PR).
2. Point the ## Translations section of docs-site/README.md to that page instead of holding the details itself.
3. Add a brief mention of docs translations to the ## Translations sections of docs/contributing.md and docs-site/developers/contributing.mdx, which currently only cover interface string translations.