Github pages for API documentation

[dsilhavy]

Use Github pages to host the API documentation of the MBMS-Modem. Similar to:

• https://5g-mag.github.io/rt-mbms-modem/api/
• https://5g-mag.github.io/rt-mbms-modem/

[dsilhavy]

Enabled gh-pages for the project and linkes to /docs folder of gh-pages branch per default:

• https://5g-mag.github.io/rt-mbms-modem/
• https://5g-mag.github.io/rt-mbms-modem/api