Merge branch 'docs/styleguide-haml-help-links' into 'master'

Add guidelines in doc linking with HAML

## What does this MR do?

Add guidelines and examples of HAML links to
the buil-in help GitLab exposes under /help.

See merge request !7798

doc/development/doc_styleguide.md

@@ -113,6 +113,77 @@ merge request.
   add an alternative text: `[identifier]: https://example.com "Alternative text"`
   that appears when hovering your mouse on a link
 
+### Linking to inline docs
+
+Sometimes it's needed to link to the built-in documentation that GitLab provides
+under `/help`. This is normally done in files inside the `app/views/` directory
+with the help of the `help_page_path` helper method.
+
+In its simplest form, the HAML code to generate a link to the `/help` page is:
+
+```haml
+= link_to 'Help page', help_page_path('user/permissions')
+```
+
+The `help_page_path` contains the path to the document you want to link to with
+the following conventions:
+
+- it is relative to the `doc/` directory in the GitLab repository
+- the `.md` extension must be omitted
+- it must not end with a slash (`/`)
+
+Below are some special cases where should be used depending on the context.
+You can combine one or more of the following:
+
+1. **Linking to an anchor link.** Use `anchor` as part of the `help_page_path`
+   method:
+
+    ```haml
+    = link_to 'Help page', help_page_path('user/permissions', anchor: 'anchor-link')
+    ```
+
+1. **Opening links in a new tab.** This should be the default behavior:
+
+    ```haml
+    = link_to 'Help page', help_page_path('user/permissions'), target: '_blank'
+    ```
+
+1. **Linking to a circle icon.** Usually used in settings where a long
+   description cannot be used, like near checkboxes. You can basically use
+   any font awesome icon, but prefer the `question-circle`:
+
+    ```haml
+    = link_to icon('question-circle'), help_page_path('user/permissions')
+    ```
+
+1. **Using a button link.** Useful in places where text would be out of context
+   with the rest of the page layout:
+
+    ```haml
+    = link_to 'Help page', help_page_path('user/permissions'),  class: 'btn btn-info'
+    ```
+
+1. **Underlining a link.**
+
+    ```haml
+    = link_to 'Help page', help_page_path('user/permissions'), class: 'underlined-link'
+    ```
+
+1. **Using links inline of some text.**
+
+    ```haml
+    Description to #{link_to 'Help page', help_page_path('user/permissions')}.
+    ```
+
+1. **Adding a period at the end of the sentence.** Useful when you don't want
+   the period to be part of the link:
+
+    ```haml
+    = succeed '.' do
+      Learn more in the
+      = link_to 'Help page', help_page_path('user/permissions')
+    ```
+
 ## Images
 
 - Place images in a separate directory named `img/` in the same directory where