API docs search API

Created by: slimsag

Helps #21938

Depends on:

• https://github.com/sourcegraph/sourcegraph/pull/26973
• https://github.com/sourcegraph/sourcegraph/pull/26954
• https://github.com/sourcegraph/sourcegraph/pull/26771
• https://github.com/sourcegraph/sourcegraph/pull/26913
• https://github.com/sourcegraph/sourcegraph/pull/26736
• https://github.com/sourcegraph/sourcegraph/pull/26487
• https://github.com/sourcegraph/sourcegraph/pull/26486
• https://github.com/sourcegraph/sourcegraph/pull/26485
• https://github.com/sourcegraph/sourcegraph/pull/26484
• https://github.com/sourcegraph/sourcegraph/pull/26461