Add Cirrus CI config file

[ygy]

Summary

Perform a complete doc tree build test for Github mirrors and pull
requests. We can enable this in our Github mirror, and anyone who
pushes the repo to Github will have CI functionality for this repo as
well as any Pull Requests for it.

src is already doing this: https://github.com/freebsd/freebsd/blob/master/.cirrus.yml

Test Plan

End results should look like this:

https://github.com/yzgyyang/freebsd-doc/pull/7
https://cirrus-ci.com/task/5775079176929280

[lwhsu]

.cirrus.yml

5

I'm not sure what's the "standard" or recommended build platform of doc, but perhaps latest stable or release? Maybe we want freebsd-12-2-snap or freebsd-12-2 here.

[ygy]

Update to use 12.2-STABLE.

[lwhsu]

.cirrus.yml

2

Oh, and I believe we can drop $FreeBSD$ tag in git era.

[lwhsu]

LGTM, wait for doc approval.

[bcr]

OK from doc.

[ygy]

Closed by https://cgit.freebsd.org/doc/commit/?id=2cd339f0e95f34459fccea150161ba3d2ed3c22b.