Add reference topic with search and scope info to search page docs

Description

There are 3 search types (basic, advanced, exact code search), 3 search levels (project, group, global) and multiple scopes (code, commits, wiki, issues, merge requests, milestones, users, epics, etc). Multiple settings and feature flags control when things are visible to the user when doing searches but this isn't documented well.

Proposal

create a reference topic in the main Searching in GitLab page

Basic search

level commit code wiki notes other scopes searchable (projects, issues, merge requests, milestones, users)
global no no no no

yes

* can be disabled in application settings

group no no no no yes
project yes yes yes yes yes

Advanced search

level limiting enabled

advanced_global_search_for_limited_indexing

feature flag setting

code scopes searchable? non-code scopes searchable?
global no

yes

* can be disabled in application settings

yes

* can be disabled in application settings

group no yes yes
project no yes yes
global yes disabled no yes
global yes enabled

yes (in limited namespaces)

* can be disabled in application settings

yes (in limited namespaces)

* can be disabled in application settings

group yes disabled / enabled yes yes
project yes disabled / enabled yes yes

I don't think we should include a table about Zoekt on this page, there are a lot more feature flags and I'm not sure the below table captures them all.

Exact code search

level

advanced_global_search_for_limited_indexing

feature flag setting

zoekt_cross_namespace_search feature flag setting

code searchable?
global enabled disabled no
global disabled disabled no
global enabled enabled

yes

* can be disabled in application settings

global disabled disabled no
group enabled/disabled enabled/disabled yes
project enabled/disabled enabled/disabled yes

The following discussion from !184946 (merged) should be addressed:

Edited by Terri Chu