[ADD] contributing/development: add a guide to contribute to the codebase

In addition, this commit extracts the git guidelines from the coding
guidelines. This helps link the git guidelines and unclutter the coding
guidelines, which should focus on only the programming languages.

task-2897123

Part-of: odoo/documentation#3260

content/contributing/install_git.rst

@@ -0,0 +1,20 @@
+`Install Git <https://git-scm.com/book/en/v2/Getting-Started-Installing-Git>`_. It is a command-line
+(:dfn:`a text interface`) tool that allows tracking the history of changes made to a file and, more
+importantly, working on different versions of that file simultaneously. It means you do not need to
+worry about overwriting someone else’s pending work when making changes.
+
+Verify that the installation directory of Git is included in your system's `PATH` variable.
+
+.. tabs::
+
+   .. group-tab:: Linux and macOS
+
+      Follow the `guide to update the PATH variable on Linux and macOS
+      <https://unix.stackexchange.com/a/26059>`_ with the installation path of Git (by default
+      :file:`/usr/bin/git`).
+
+   .. group-tab:: Windows
+
+      Follow the `guide to update the PATH variable on Windows
+      <https://www.howtogeek.com/118594/how-to-edit-your-system-path-for-easy-command-line-access/>`_
+      with the installation path of Git (by default :file:`C:\\Program Files\\Git`).