Update site description with mission statement

[valencik]

The mission statement taken from:
https://typelevel.org/blog/2022/04/01/call-for-steering-committee-members.html

[valencik]

Recently I heard someone unfamiliar with our community try to summarize Typelevel as a group that makes compiler plugins.
We're certainly a lot more than that, and I think this mission statement from the call for committee members does a better job introducing Typelevel as a community.

[valencik]

Since this is so front and center, I'm going to tag the whole group @typelevel/steering :)

[jducoeur]

I might leave off the "community of", but I'm of mixed minds -- this works reasonably well, and is a clear improvement IMO.

[SethTisue]

This is of course already mergeable as a major improvement, yet I can't resist the urge to brainstorm more variants. Maybe:

Typelevel is a community working together since 2013 to foster an inclusive, welcoming, and safe environment around functional programming in Scala. Our ecosystem of libraries is mature and widely adopted.

[valencik]

This is of course already mergeable as a major improvement, yet I can't resist the urge to brainstorm more variants. Maybe:

Typelevel is a community working together since 2013 to foster an inclusive, welcoming, and safe environment around functional programming in Scala. Our ecosystem of libraries is mature and widely adopted.

@SethTisue I like that expanded version. It makes me think I should follow this up with an edit to the actual About page and provide some more details/context. I think I'll keep the "description" tagline this shorter version though.
Thanks! (I'll be sure to ping you when I open the other PR :)