Add Autofill to data entry flow

[steverep]

Proposed change

Documents how integration authors can enable browser autofill in their data entry flows. Requires merging home-assistant/core#81060.

Because it didn't really fit well into the existing doc, I took the liberty of rearranging the "Show Form" section and organizing into sub-headings. I think it reads more clearly now.

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: Add autocomplete to text selector core#81060

[bdraco]

It would be easier to review this if it was split into multiple PRs

[steverep]

If you look at the 2 commits separately it's probably a lot easier to review. The first just literally moves text around to avoid repetition about translations and add sub-sections. And then I organized the keys into a small table. There's no real new text.

The second adds the new autofill section.

You're right though. I probably would have split it up if I wrote it in that order. 🙃