Support repo code search without setting up an indexer (#29998)

By using git's ability, end users (especially small instance users) do
not need to enable the indexer, they could also benefit from the code
searching feature.

Fix #29996


![image](https://github.com/go-gitea/gitea/assets/2114189/11b7e458-88a4-480d-b4d7-72ee59406dd1)


![image](https://github.com/go-gitea/gitea/assets/2114189/0fe777d5-c95c-4288-a818-0427680805b6)

---------

Co-authored-by: silverwind <me@silverwind.io>

administration/repo-indexer.en-us.md

@@ -17,6 +17,12 @@ menu:
 
 # Repository indexer
 
+## Builtin repository code search without indexer
+
+Users could do repository-level code search without setting up a repository indexer.
+The builtin code search is based on the `git grep` command, which is fast and efficient for small repositories.
+Better code search support could be achieved by setting up the repository indexer.
+
 ## Setting up the repository indexer
 
 Gitea can search through the files of the repositories by enabling this function in your [`app.ini`](administration/config-cheat-sheet.md):

installation/comparison.en-us.md

@@ -87,6 +87,9 @@ _Symbols used in table:_
 | Git Blame                                   | ✓                                                   | ✘    | ✓         | ✓         | ✓         | ✓         | ✓            | ✓            |
 | Visual comparison of image changes          | ✓                                                   | ✘    | ✓         | ?         | ?         | ?         | ✘            | ✘            |
 
+- Gitea has builtin repository-level code search
+- Better code search support could be achieved by [using a repository indexer](administration/repo-indexer.md)
+
 ## Issue Tracker
 
 | Feature                       | Gitea                                               | Gogs | GitHub EE | GitLab CE | GitLab EE | BitBucket | RhodeCode CE | RhodeCode EE |