Update Docusaurus to v3

[TomBrien]

Proposed change

Updates Docusaurus to V3.

Couple of things that are probably not as friendly as before:

• Links where we just want to show the address can't be given as <https://example.com> seem to need to use [https://example.com](https://example.com)
• Need to escape more characters. E.g., <= needs be \<=
• HTML tags need us to be careful with line breaks to avoid being broken by an implied <p> in MDX 2. See here

Type of change

• Document existing features within Home Assistant
• Document new or changing features which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Changes to the backend of this documentation
• Removed stale or deprecated documentation

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: