Restructure documentation. Now the documentation has installation, administration, usage, development, contributing the 5 main parts (#23629)

- **Installation**: includes how to install Gitea and related other
tools, also includes upgrade Gitea
- **Administration**: includes how to configure Gitea, customize Gitea
and manage Gitea instance out of Gitea admin UI
- **Usage**: includes how to use Gitea's functionalities. A sub
documentation is about packages, in future we could also include CI/CD
and others.
- **Development**: includes how to integrate with Gitea's API, how to
develop new features within Gitea
- **Contributing**: includes how to contribute code to Gitea
repositories.

After this is merged, I think we can have a sub-documentation of `Usage`
part named `Actions` to describe how to use Gitea actions

---------

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

docs/content/doc/administration/search-engines-indexation.en-us.md

@@ -0,0 +1,38 @@
+---
+date: "2019-12-31T13:55:00+05:00"
+title: "Advanced: Search Engines Indexation"
+slug: "search-engines-indexation"
+weight: 30
+toc: false
+draft: false
+menu:
+  sidebar:
+    parent: "administration"
+    name: "Search Engines Indexation"
+    weight: 60
+    identifier: "search-engines-indexation"
+---
+
+# Search engines indexation of your Gitea installation
+
+By default your Gitea installation will be indexed by search engines.
+If you don't want your repository to be visible for search engines read further.
+
+## Block search engines indexation using robots.txt
+
+To make Gitea serve a custom `robots.txt` (default: empty 404) for top level installations,
+create a file called `robots.txt` in the [`custom` folder or `CustomPath`]({{< relref "doc/administration/customizing-gitea.en-us.md" >}})
+
+Examples on how to configure the `robots.txt` can be found at [https://moz.com/learn/seo/robotstxt](https://moz.com/learn/seo/robotstxt).
+
+```txt
+User-agent: *
+Disallow: /
+```
+
+If you installed Gitea in a subdirectory, you will need to create or edit the `robots.txt` in the top level directory.
+
+```txt
+User-agent: *
+Disallow: /gitea/
+```