Update `xml5ever/README.md` to reflect current API documentation

[hasezoey]

Currently xml5ever/README.md refers in

If you need docs checkout either API docs or run cargo docs to generate documentation.

to https://ygg01.github.io/docs/xml5ever/xml5ever/index.html, which hosts a very outdated API documentation

also, it seems the README is generally outdated (like using function parse that i cant find anywhere, maybe needs to be updated to parse_document?)

[atouchet]

I've removed some outdated info and updated the API documentation links in #532 and #534 but the rest of the Readme information and code example probably still need updates.