API docs: turn search indexing off-by-default for the release

Created by: slimsag

API docs search indexing has caused LSIF queue growth / increased LSIF upload times, so disable by default for this release and until it stabilizes.

This will fix the immediate issue on Sourcegraph.com as it effectively disables all recent API docs changes.

Helps #26292

Signed-off-by: Stephen Gutekanst stephen@sourcegraph.com