Update readme files

[gtrevisan]

• modify contributing guide,
• add badges to readme,
• add security policy,
• [x] fully revamp the readme -- @crea-psfc,
• [x] other rough pass at the docs -- @amdecker.

preview: https://github.com/MIT-PSFC/disruption-py/blob/update-readmes/README.md

[amdecker]

Given the readme mentions SPARC, I'll add that at CFS there's encouragement to use the word "device" or "machine" (and later, hopefully, "power plant") instead of "reactor" to have a strong differentiation between fusion projects and fission projects. https://github.com/MIT-PSFC/disruption-py/blob/74d4c150eae0083895f38253d741a12caf61cb8c/README.md?plain=1#L29

[gtrevisan]

@amdecker how should we solve the duplicated information between README.md and docs/index.md? copy/paste? (actually, IMHO we should use the mkdocs webpage with caution for now -- eg for automated API documentation and any technical Q&A...)

[amdecker]

@amdecker how should we solve the duplicated information between README.md and docs/index.md? copy/paste? (actually, IMHO we should use the mkdocs webpage with caution for now -- eg for automated API documentation and any technical Q&A...)

One idea would be we could add this to our docs/index.md --8<-- "README.md" which would insert the contents of the readme file into the docs. If we do that, we would need to change a few of the relative links to be absolute.

[gtrevisan]

@amdecker is this good to go wrt the docs?