Revise and automate docs with sphinx and host on readthedocs

[lukpueh]

Description of issue or feature request:
Revise docstrings of API functions regarding contents and format (use Google Style docstring as suggested in secure-systems-lab/code-style-guidelines#20), akin to what in-toto did in in-toto/in-toto#369.

Needs coordination with #270

Current behavior:

• No clear API
• Docstrings use custom secure systems lab format

Expected behavior:

• Clear API via Revise module architecture and define clear API modules and functions #270
• Use Google-style quasi standard docstring format (see napoleon sphinx extension)
• Revise doc contents (aim for conciseness)

[MVrachev]

Please note that when we update the docstrings of API functions we should update the
documentation regarding the securesystemslib.keys.create_signature() and the following comment

securesystemslib/securesystemslib/keys.py

Line 709 in a7d3668

# Continue to support keytypes of ecdsa-sha2-nistp256 and ecdsa-sha2-nistp384

because we are not actually supporting ecdsa-sha2-nistp384.
If you read securesystemslib.ecdsa_keys.create_signature():

securesystemslib/securesystemslib/ecdsa_keys.py

Line 173 in a7d3668

def create_signature(public_key, private_key, data, scheme='ecdsa-sha2-nistp256'):

you will see we support only support ecdsa-sha2-nistp256.