Unify doc links to use paths relative to doc folder (#24979)

Changes:

1. Use uniform links types relative to doc folder (start with `doc/`)
2. According to [docusaurus
links](https://docusaurus.io/docs/markdown-features/links), if `<a>` is
used, the `href` is resolved as URL location, but not file location. So
need to use `[text]({{< relref "path" >}})` instead.

docs/content/doc/installation/from-source.en-us.md

@@ -35,8 +35,7 @@ executable path, you will have to manage this yourself.
 
 **Note 2**: Go version {{< min-go-version >}} or higher is required. However, it is recommended to
 obtain the same version as our continuous integration, see the advice given in
-<a href='{{< relref "doc/development/hacking-on-gitea.en-us.md" >}}'>Hacking on
-Gitea</a>
+[Hacking on Gitea]({{< relref "doc/development/hacking-on-gitea.en-us.md" >}})
 
 **Table of Contents**