Add Code quality guidelines to TARDIS docs

[jaladh-singhal]

The main aim of these guidelines will be to explain what does good quality code in a PR expects (meaning of "maintaining code quality" section in CONTRIBUTING.md):

• code covers edge cases, exception handling where required
• easy to maintain and extend - OOP design pattern
• follows PEP8 style guide (use of black)
• more pointers can be found in this code quality guide by real-python

Code style guide (#1170) should probably be a section within it and this guide should be placed within "Contributing to TARDIS" section in https://github.com/tardis-sn/tardis/blob/master/docs/index.rst

NEED: This is pointed in CONTRIBUTING.md as a void link (#), so it is necessary to add them soon.

[jaladh-singhal]

Can use some inspiration from astropy - https://docs.astropy.org/en/latest/development/codeguide.html