Replace Flask with nginx

[nottrobin]

Fixes https://github.com/ubuntudesign/base-squad/issues/131.

Same as phone-docs: https://github.com/canonical-docs/phone-docs/pull/17

QA

Test the run script

./run clean
./run

Browse to http://localhost:8204, and check the core docs work. Go to http://localhost:8204/en/, see that you're redirected to http://localhost:8204/stable/en/ - proving that redirects.map works.

Test the Dockerfile

(You must have done ./run or ./run build first - which is why this is the second QA step)

docker build --tag security-docs --build-arg COMMIT_ID=`git rev-parse --short HEAD` .
docker run -p 80:80 security-docs  # NB: Check nothing else is running on 80, e.g. microk8s

Go to http://localhost and http://localhost/en/, check it works similarly.