Added docs for agit-setup (#21027)

This PR Adds Documentation for the new Agit feature. Agit allows creating PR directly while pushing code.

Close #21018

Co-authored-by: CypherpunkSamurai <CypherpunkSamurai@users.noreply.github.com>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: a1012112796 <1012112796@qq.com>

docs/content/doc/usage/agit-support.en-us.md

@@ -0,0 +1,47 @@
+---
+date: "	2022-09-01T20:50:42+0000"
+title: "Usage: Agit Setup"
+slug: "agit-setup"
+weight: 12
+toc: false
+draft: false
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Agit Setup"
+    weight: 12
+    identifier: "agit-setup"
+---
+
+# Agit Setup
+
+In Gitea `1.13`, support for [agit](https://git-repo.info/en/2020/03/agit-flow-and-git-repo/) was added.
+
+## Creating PRs with Agit
+
+Agit allows to create PRs while pushing code to the remote repo. \
+This can be done by pushing to the branch followed by a specific refspec (a location identifier known to git). \
+The following example illustrates this:
+
+```shell
+git push origin HEAD:refs/for/master
+```
+
+The command has the following structure:
+
+- `HEAD`: The target branch
+- `refs/<for|draft|for-review>/<branch>`: The target PR type
+  - `for`: Create a normal PR with `<branch>` as the target branch
+  - `draft`/ `for-review`: Currently ignored silently
+- `<branch>/<session>`: The target branch to open the PR
+- `-o <topic|title|description>`: Options for the PR
+  - `title`: The PR title
+  - `topic`: The branch name the PR should be opened for
+  - `description`: The PR description
+  - `force-push`: confirm force update the target branch
+
+Here's another advanced example for creating a new PR targeting `master` with `topic`, `title`, and `description`:
+
+```shell
+git push origin HEAD:refs/for/master -o topic="Topic of my PR" -o title="Title of the PR" -o description="# The PR Description\nThis can be **any** markdown content.\n- [x] Ok"
+```