[IMP] contributing/documentation: rewrite the contribution guide

This commit restructures the contributing/documentation page to re-use
content introduced with the contributing/development page and to
display the two ways to contribute (github vs git) in content tabs
rather than sections.

The guide is simplified and updated to delegate long explanations to
other pages or websites, remove useless images and focus on getting
users ready to contribute.

task-2897123

Part-of: odoo/documentation#3265

content/contributing/documentation/changes_approved.rst

@@ -0,0 +1 @@
+Once your changes are approved, the reviewer merges them and they appear online the next day!

content/contributing/documentation/content_guidelines.rst

@@ -182,7 +182,8 @@ content. However, avoid adding too many images: it isn't necessary to illustrate
 features, and it may overload your page.
 
 .. important::
-   Don't forget to :ref:`compress your PNG files with pngquant <contributing/pngquant>`.
+   Don't forget to :ref:`compress your PNG files with pngquant
+   <contributing/documentation/first-contribution>`.
 
 .. _contributing/screenshots:
 

content/contributing/documentation/rst_guidelines.rst

@@ -10,7 +10,7 @@ Use relative links for internal URLs
 If you need to reference an internal documentation page or a file that is not sitting in the same
 directory as your current page, always make use of *relative file paths* rather than *absolute file
 paths*. An absolute file path indicates the location of the target from the root of its file tree. A
-relative file path makes use of smart notations (such as ``../`` git that redirects to the parent
+relative file path makes use of smart notations (such as ``../`` that redirects to the parent
 folder) to indicate the location of the target *relative* to that of the source document.
 
 Example