Migrate 'Why Join Typelevel' section

[valencik]

Migrate 'Why Join Typelevel' section from the governance README and into the site.

We likely want to link to this new page in the submission call to action:

Maybe change the button to read "Learn More"
And then the linked page can link to the issue templates.

[SykoDaedalus]

Hey @valencik it is my first time around here. I see that there is a PR related to the issue. I wanted to ask if still require some work done and if yes may I contribute to it?