[#334] Add content and API search

[shadcn]

API search has been requested a few times. See #289, #298 and #334.

Since api_doc is a custom entity type, core search module cannot index it.

This PR adds an experimental apigee_kickstart_search module that ships with a search page for both content (article, landing page, faq and forum) and api_doc.

apigee_kickstart_search is a drop-in replacement for the default search. It has a dependency on the search_api module.

See also the README in this PR.

[cnovak]

@arshad I think this is not going to be needed or will need to change after @arlina-espinoza finishes her changes for the API Catalog to add RBAC. The APIDoc entity is being removed and will be a content type after her change. You probably want to put this on hold until she adds a PR for that issue.

[shadcn]

@cnovak Thanks. I added a comment to the ticket. I'm going to close this PR.