[WIP] Add documentation

[EZoni]

To do

• Add minimal working setup.
• Add first draft, starting from the existing README files.

Test

cd docs
conda env create -f docs.yml
conda activate synapse-docs
make html

Follow up

• Set up continuous deployment via Read the Docs.
• Update names of the other conda environments?
  • synapse-docs (this PR).
  • gui v. synapse-gui.
  • ml-training v. synapse-ml.

[EZoni]

To discuss:

• Names (so far, all contributors plus Jean-Luc Vay).
• Order (so far, alphabetical with respect to the last name).