Add documentation to project repo

[orangegrove1955]

With patchthecode.com currently down, there is limited access to the documentation. While a zip file with HTML files is possible to use, it would be better to have the documentation available in a more easily accessible format.

This PR seeks to take the information provided in the zip file from #1397 (comment) and copy it into a new set of Markdown files contained within the project repo itself as a baseline step for improved documentation access.

It is important to note that the information contained within these new files may be out of date with respect to the current project, and so further improvements to the documentation will need to be made.

I have also included a few flags throughout these new files, to highlight areas of improvement, and to warn those using the documents that they may be out of date.

Additionally, this PR includes changes to the readme to remove external site references since they no longer work, as well as including new links to the documentation.

Finally, this PR includes a new CONTRIBUTING.md file, since this appeared to be a broken link. The file is essentially empty, and will need to be added to in future