automatically build HTML docs as part of CI

Coqdoc produces really nice output - let's automate this.
Status Job ID Name Coverage
  Build
passed #35273
1.8.0-coq-8.8

00:02:02

passed #35274
1.9.0-coq-8.9

00:02:03

failed #35275
allowed to fail
1.9.0-coq-dev

00:00:13

passed #33878
1.8.0-coq-8.8

00:02:26

passed #33880
1.9.0-coq-8.9

00:02:28

canceled #35271
allowed to fail
1.9.0-coq-dev

00:00:11

failed #33879
allowed to fail
1.9.0-coq-dev

00:00:14

 
  Process
passed #35276
doc

00:00:28

passed #33881
validate

00:04:18

canceled #35272
doc

failed #35270
doc

passed #33882
doc

00:00:23

 
Name Stage Failure
failed
1.9.0-coq-dev Build There is an unknown failure, please try again
File "./util/seqset.v", line 19, characters 42-52:
Error: The reference mkPredType was not found in the current environment.

make[1]: *** [util/seqset.vo] Error 1
make[1]: *** Waiting for unfinished jobs....
Makefile:658: recipe for target 'util/seqset.vo' failed
make: *** [all] Error 2
Makefile:320: recipe for target 'all' failed
ERROR: Job failed: exit code 1