Search on the documentation and extensions website

[agallou]

algolia has provided unified search (on our multiple websites) via algolia docsearch.

This PR:

• adds docsearch (search on extension and documentation)
• make some changes to the display of the results to made them more readable
• removes the old way of indexing the site

It will look like this when the input is not focused:

It will look like this when the input is focused and a search done:

[Grummfy]

cela me semble bon, mais je n'ai jeté qu'un coup d'oeil rapide