Add CNAME file for MkDocs and gh-pages

[n-vr]

MkDocs requires you to put a CNAME file in your docs dir. This is most likely what caused the site not to work.

This is supposed to fix the problems talked about in #56

[henriterhofte]

Did not solve it, despite the job log saying:

Run mkdocs gh-deploy --force
INFO     -  Cleaning site directory
INFO     -  Building documentation to directory: /home/runner/work/twomes-generic-esp-firmware/twomes-generic-esp-firmware/site
INFO     -  Documentation built in 0.46 seconds
WARNING  -  Version check skipped: No version specified in previous deployment.
INFO     -  Copying '/home/runner/work/twomes-generic-esp-firmware/twomes-generic-esp-firmware/site' to 'gh-pages' branch and pushing to GitHub.
To https://github.com/energietransitie/twomes-generic-esp-firmware
 + 881ca6b...fcb5[7](https://github.com/energietransitie/twomes-generic-esp-firmware/actions/runs/3264444069/jobs/5365098452#step:5:8)3f gh-pages -> gh-pages (forced update)
INFO     -  Based on your CNAME file, your documentation should be available shortly at: http://www.energietransitiewindesheim.nl/
INFO     -  NOTE: Your DNS records must be configured appropriately for your CNAME URL to work.