[Docs] Tweaks and improvements

[javiereguiluz]

This PR includes the following:

• Some minor formatting and typo fixes
• Change the <h3> underline to ~~~ instead of ^^^ to comply with Symfony Docs practices
• Change the <h4> underline to ... instead of ''' to comply with Symfony Docs practices
• Reindent all code blocks (they used 3 white spaces but we need 4)
• Use code-block:: terminal instead of code-block:: bash when we want to display some console command
• Use .. note:: and .. caution:: admonitions
• Fix the syntax of some internal references
• Display the "Before Symfony 5.3" and "When using Symfony 5.3 or higher" texts as headings to not confuse readers

[chalasr]

Thanks a ton, Javier!