add building documentation

this resurrects an old .gitlab-ci.yml, because I thought that's a
good idea event if it is ignored by github.

.gitlab-ci.yml

+test:
+    image: python:3.8-slim
+    script:
+        - python3 -m ebpfcat.ebpf_test -v Tests
+
+docs:
+    image: tk0miya/sphinx-html
+    script:
+        - sphinx-build -b html -d _build/doctrees . _build/html
+        - eval $(ssh-agent)
+        - echo "$SSHKEY" | ssh-add -
+        - ssh -o UserKnownHostsFile=known_hosts wp86team@exflqr30526 "./doc-prepare ec_$CI_COMMIT_REF_NAME"
+        - scp -o UserKnownHostsFile=known_hosts -r _build/html wp86team@exflqr30526:html/ec_$CI_COMMIT_REF_NAME

known_hosts

+exflqr30526,131.169.220.113 ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCcPp1A0EeC72+tSXKqfVfIlxAKq3Bp7CA2LfSbYD9hw4SKp6UhGcM8R5eQ5wnoMCoJ4IwqMqA0Mi24K9kgQxpAao6GOh3Yx+YznLtKGZRx3EYUM+9s+fDEzmZvymZF5aBqfwx6j1aUiSKciVbo/DNjU0PxuTjliicZqqOaaPVZPMHJA9GZuvwvfxcaFp8fDWAMSRKr01rihcJZljXLe8ZLBZTYt8i+W561WrgZiGztLxaBMT9o+MbEkHsUor5xBidW6NOH1TkQw3wiVp0vl6/ca9VskMIr3OuWCdce6YqR3GC3wBJMMgUeFxyKr3t0k1b4Voxbcy1dh0ObaFPniBAv