No search form on developer handbooks

[SergeyBiryukov]

Previously: #1739.

The code reference has an expanding search form at the top of the page:

• ​https://developer.wordpress.org/reference/hooks/

Plugin and theme developer handbooks don't have it, same goes for REST API and CLI:

• ​https://developer.wordpress.org/plugins/
• ​https://developer.wordpress.org/themes/
• ​https://developer.wordpress.org/rest-api/
• ​https://developer.wordpress.org/cli/commands/

Only the general WordPress.org search in header is there, not specific to these sites.

Introduced in [6512], which reverted [3282]. The commit message mentions an imminent switch to search widget, which apparently has never happened.

[coffee2code]

In 7752:

developer.wordpress.org: Don't hide search form when it appears in sidebar, as it does for handbooks.

Fixes #3859.