Update docs with minor improvements for add-on discoverability

[CryHavoc21]

I wanted to make some changes to the docs to help with discoverability of some features, most notably for add-ons. I'm not an expert on markdown, so the links for now are URL links instead of links between markdown pages.

Summary of changes:

README.md: added a link to the github page (as far as I know, this is the page that generates https://gp2040-ce.info/, and there was no link back to the github repo) and a direct link to Turbo functionality under the features list. In future this could be expanded to direct link all the features to their documentation, but I'm leaving it for now so that this PR can be incremental

docs/usage.md: I pulled the paragraph from InfraredAces on discord from our discussion to replace the "unfinished docs" section on turbo and analog stick emulation, but with extra links to turbo and analog stick add-ons (to preserve the link from this page to that concept)

docs/web-configurator-add-ons.md: Reworded the "experimental" phrasing, since from discussion with InfraredAces, not all add-ons should be considered experimental. Some trailing spaces were also removed.

[InfraredAces]

Corrections on typos and usage of markdown links referencing sections in other documents