docs: update rich text to HTML conversion documentation (#12465)

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.

docs/rich-text/converting-plaintext.mdx

@@ -6,7 +6,7 @@ desc: Converting between lexical richtext and plaintext
 keywords: lexical, richtext, plaintext, text
 ---
 
-## Converting Richtext to Plaintext
+## Richtext to Plaintext
 
 Here's how you can convert richtext data to plaintext using `@payloadcms/richtext-lexical/plaintext`.