Doc revisions1

[matteius]

Thank you for contributing to Pipenv!

The issue

The docs are long in need of work -- this is revision1 which tackles cleaning up the contents from the Basics and Advanced sections and splitting them apart. There were some bits left in advanced that I wasn't sure where to put them, so I left that section but renamed it to be other since most of the advanced information moved to other sections. I also added some context on the pipenv update and pipenv upgrade commands rework.

The fix

This is not the end-all be-all fix to the docs, no this is just progress. I plan to iterate on this more over time -- splitting apart advanced as well.

I was getting frustrated with rst files and found I was having better luck making the new sections markdown.

• Removes the super old animation (we can make a new one, but I didn't want that old one holding us back longer)
• Removes references to python3.6 and python 2 -- also just old weird misinformation in general removed.
• Language updated to have a cohesive voice.

I am sure I missed some things and its definitely not perfect, but I think its an improvement.

The checklist

• Associated issue
• A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.