Move semiotic-docs to semiotic repo

This PR needs to be merged with Merge strategy to preserve git history

In this PR I migrate docs from semiotic-docs (semi-together branch) to docs folder in this repo

Things I performed to move semiotic-docs while preserving git history:

# get latest version of docs
git clone https://github.com/nteract/semiotic-docs.git
cd semiotic-docs
git checkout semi-together

# move all the contents to a dedicated docs folder
mkdir docs 
git mv <EVERY FILE/FOLDER FROM ROOT> docs
git add .
git commit -m "move semiotic-docs to docs folder"

# switch to local copy of semiotic repo and create this branch
cd ../semiotic
git checkout -b mergeDocs

# setup local git origin and fetch it
git remote add semiotic-docs ../semiotic-docs
git fetch semiotic-docs
git branch semiotic-docs remotes/semiotic-docs/semi-together
git merge semiotic-docs --allow-unrelated-histories

Thus, we get a branch that includes docs folder with contents from semiotic-docs repo and all its git history preserved.

Closes #572

nteract / semiotic

Move semiotic-docs to semiotic repo #578