Update and improve documenation

[aidanjungo]

To simplify the writing, updating and the understanding of CEASIOMpy documentation, it has been decided to write it only as Markdown file in the Gihhub repo with correct links and references. The ReadTheDocs will be remove, it became a bit to complex to maintain a mix of RST and MD file In the future, we could use a tool like pydoc-markdown to generate a Github page directly from the Markdown files

TODO:

• [x] Re-write the README in MD
• [x] How to contribute page should be sync with CONTRIBUTING.md
• [x] Installation link to README
• [ ] Update test case examples
  • [x] Test case 1
  • [x] Test case 2
  • [ ] Test case 3
  • [ ] Test case 4
• [x] Move documentation of each module in the module directory
• [x] Add explanation for __specs__.py
• [x] Save the libpng12 file and other documents (like pfd's) in a new documents folder at the root of CEASIOMpy
• [ ] Add references (Thesis and paper)
• [x] Check that all the info from the old doc has been include.
• [x] Remove old doc
• [x] Remove dependencies for the doc from environment.yml
• [x] Remove ReadTheDoc building
• [x] Remove link to the doc from Github
• [x] Remove doc Badge from README
• [x] Remove link to the doc from CEASIOMpy website

[aidanjungo]

A big part of this issue has been done in #164, other smaller PR will be open to do the rest.