build: Build documentation from source during distcheck

This verifies that we can.

Signed-off-by: Simon McVittie <smcv@debian.org>
2 jobs for test-autotools in 4 minutes and 52 seconds (queued for 19 seconds)
Status Job ID Name Coverage
  Test
passed #10445424
build_autotools

00:04:32

passed #10445423
build_stable

00:02:05