Fix links with trailing `.html` on README

[gui machiavelli]

There are currently several links in the README pointing to a page in the new docs website, but using an .html extension. These links don't work and lead users to 404 pages.

We need to go through all links to the documentation and remove the .html extension. This seems to be common on links containing URI fragments (meilisearch.com/docs/page.html#heading-text).

[cvermand]

Thanks a lot for raising this :) We'll work on it to fix these links