Remove What Is MeiliSearch

[dichotommy]

learn/what_is_meilisearch is a directory containing at this time 7 pages loosely connected by the fact that none particularly belong on the docs site (except perhaps learn/what_is_meilisearch/telemetry.md).

The main reason we have chosen to keep these pages until now is because our main website (AKA landing page internally) hasn't been updated in some time and lacks aspects of a fully developed product site, such as a features page, a detailed comparison to competitors, and a contact page (doesn't even contain a link to our Slack, AFAIK). Since the docs site is already highly visited, it has operated as a convenient stand-in.

A new version of the main site is in the works and should be released soon; once it has and we're sure there will be no significant content loss, we can likely remove all of the pages present in learn/what_is_meilisearch with the exception of learn/what_is_meilisearch/telemetry.md.

[dichotommy]

Some time after the new landing page has been released, we expect to add a section to the main website (not docs site) called the resource center. Something that remains to be defined is the types of content that belong on the docs, vs. on the main landing page or resource center. This is how I see it:

• Marketing-type content, product info, contact info -> landing page
• User-created content, guest posts, guides and tutorials relating to software that we don't maintain -> resource center
• Content written by the Meili team about how MeiliSearch works and how to use it -> docs

This should be preferable because the docs will have to do much less heavy lifting and can concentrate on what we do best (documenting + teaching, not so much "selling" the software to new users or updating guest posts).

[dichotommy]

Superseded by #1875