Migrate to Docsearch v3

Migrate to https://docsearch.algolia.com/docs/docsearch-v3/

How to: https://docsearch.algolia.com/docs/migrating-from-legacy

Closes #1120 (closed)

Follow-up #1125 (closed)

Review app http://axil-algolia-docsearch-v3.178.62.207.141.nip.io/

• Test that the search dropdown works in the review app. Note that the results will point to docs.gitlab.com and not the review app URL, this is fine.

before	after