Sphinx documentation for stdlib specs

[jalvesz]

Proposal to start putting in place a Sphinx based documentation for stdlib. Relates to https://github.com/fortran-lang/stdlib/issues/697

It already covers the documentation in the specs. Needed a change in the way of including external files for the examples

TODO:

• [ ] How to enable embedding or linking the FORD generated dev doc to the Sphinx-generated user doc?
• [ ] Polish deployment
• [ ] Change welcome page? currently using the README content

Building locally to test:

cd doc
python -m pip install -r requirements.txt
make html