Put some platform-specific instructions under tabs

[veracioux]

I had a minute, so I decided to de-clutter some pages using the pymdownx.tabbed
extension, already listed as a dependency in mkdocs.yml. I think this makes the
instructions more readable.

[mmahmoudian]

Considering the limitations of mkdocs esp. about integrating the landing page and etc., I was thinking of moving the website to Zola static site generator so that we can have a better design. This was basically because of flameshot-org/flameshot#1710 which @Correct-Syntax kindly made a landing page for us.

Using the pymdown seems to be specific to mkdocs, and deviating from the standard markdown format would make the website more and more addicted to mkdocs environment.

Alternatively, if you know a way to force-fed the mkdocs a landing page, please let us know as it would relieve some burden.

[veracioux]

@mmahmoudian Ok, I didn't know that.

Alternatively, if you know a way to force-fed the mkdocs a landing page, please let us know as it would relieve some burden.

Unfortunately, I don't know much about mkdocs. This PR was my getting started with it :)