change package name to "tetra"

[nerdoc]

• substantial documentation update
• MkDocs as base
• readthedocs config

fixes #54
fixes #37

[nerdoc]

For merging the following conditions must be met:

• tetra pypi package must transferred by @TheCulliganMan to @nerdoc
• tetra readthedocs package tetra must be also transferred to nerdoc
• @samwillis must add a readthedocs webhook to tetra settings or grant permissions to @nerdoc
• @samwillis must upload tetraframework==0.0.6 to pypi as transition package or grant @nerdoc permissions to do that.
• A transition tetraframework=0.0.5 package must be created on pypi which depends on tetra>=0.1.0
• A tetra>0.1.0 package must be created on pypi

[TheCulliganMan]

Hey @nerdoc, thank you for the reminder. I added you as maintainer on both of these. You should have emails sent over. If this gives you permission to remove me, go for it, once you accept, i'll gladly remove myself as well.

[nerdoc]

@TheCulliganMan Thank you very much, really appreciate your friendliness and openness. That can't be taken for granted. Hope sometimes you get something back for that from somewhere. If you need anything I can do for you, anytime.
So I will remove you from the two projects, and will advance with the "tetraframework" & rename it to "tetra" then.
Thanks again.

[TheCulliganMan]

Thank you! It was nice meeting you and glad I could help!