Add gitlab-ci integration and Makefile

Makefile:
Add Makefile, introducing a simple `make lint` target which runs
rstcheck against all .rst files.

.gitlab-ci.yml:
Add GitLab CI integration, with a simple lint target, that calls `make
lint` in an environment with the required dependencies.
1 job for rfc_process in 10 seconds (queued for 3 seconds)
latest
Status Name Job ID Coverage
  Lint
passed rstcheck #16430

00:00:10