Migrate Algolia DocSearch

[aaronweissler]

As of https://docusaurus.io/blog/2021/11/21/algolia-docsearch-migration :
"Docusaurus site owners should upgrade their configuration with their new credentials by February 1, 2022, existing search indexes will be frozen and become read-only after this date."

Although to me it seems like this has not actually been enforced yet, since sections added after this date are also in the search using the old API key, it probably should be done to be safe for the future.