[docs] use Find search in Docs

[qbey]

Feature Request

Allow Docs to use Find when searching a document:

• Update the Docs indexation backend to add "path" attribute when indexing
• Update the Docs search endpoint to add "path" to requested search when user want to search only in sub-docs
• Add a feature flag on Docs to be able to use Find only for specific users
• Update Docs frontend to use the flag and the new search endpoint

[mascarpon3]

integrate Find:
suitenumerique/docs#1834
feature flags:
suitenumerique/docs#1897