documentation: gh-pages canonical link

[Florent-Bouisset]

When searching through a search engine "RxPlayer documentation" the first results are from an old version 3.5. The most referenced page for the documentation should be the current version e.g: https://developers.canal-plus.com/rx-player/doc/api/Overview.html

This PR add a tag link rel="canonical" in the metadata to inform that this page is duplicated. https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls?hl=fr#rel-canonical-link-method This tag is only used by search engine for referencing. So the documentation of all versions remains accessible with a direct URL.

Other documentation (such as react) do this: https://18.react.dev/ has <link rel="canonical" href="https://react.dev/">

From doc version 3.5 to 3.26, I have referenced to the equivalent page in current version: Ex: https://developers.canal-plus.com/rx-player/versions/3.21.1/doc/pages/api/loadVideo_options.html => https://developers.canal-plus.com/rx-player/doc/api/Loading_a_Content.html

For pages that had no equivalent in current version, I have left them unchanged.

For doc version > 3.26, I have only referenced the page Welcome.html to the current one because there is a lot of pages in the new doc version, and that would have taken much more time.