Fixes#8168, #8277
The fact that `lexicalHTMLField` doesn't work with live preview was
already clarified at the beginning of the page. I mentioned it again in
the dedicated section because it seems there was still confusion.
Also, I reordered and hierarchized the headings correctly. The
introduction said there were two ways to convert to HTML, but there were
four headings with the same level. I also made the headings a little
shorter to make the table of contents easier to parse.
- Introduces a new lexical => plaintext converter
- Introduces a new lexical <=> markdown converter
- Restructures converter docs. Each conversion type gets its own docs
pag
