docs(tvix): create Welcome + Contributing sections r/8317

A lot of this content needs to be duplicated from the monorepo’s root
directory. However, some of the content was not relevant or could have
been more condensed in descriptions or split up for consumption that
also matched the styles of some other library documentation I had been
looking at for examples. I wanted to add the new access to Admonish
callouts but also aimed at the readablitiy in one’s text editor for the
user that needs to `cat` the docs.

In the future, a “Introduction” page would be nice to add to “Welcome”,
but I don’t think that is as clear.

Change-Id: Ibfc169c5fc3501d7eeb9a76423c6f64cd68897aa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11869
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

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