Misc doc updates (mostly around running tests)

[jfly]

This adds and documents a workaround for
#4909. It feels a bit weird to
pretend to be CI just to run the test suite, though. Maybe we can do
something about that later.

I ran into a lot of trouble trying to get the tests to run when working
on #4908, and that was largely
because the instructions in this CONTRIBUTING.md file seem to have
rotted.

1. The bit about "can be run very simply" is bogus. It's
   unfortunately not that simple right now.

2. make test (the docker approach) fails for me with this error:

   $ make test
   docker-compose up
   [+] Running 1/0
    ⠿ Container pipenv-pipenv-tests-1  Recreated                                                                  0.1s
   Attaching to pipenv-pipenv-tests-1
   pipenv-pipenv-tests-1  | Collecting certifi
   pipenv-pipenv-tests-1  |   Downloading https://files.pythonhosted.org/packages/37/45/946c02767aabb873146011e665728b680884cd8fe70dde973c640e45b775/certifi-2021.10.8-py2.py3-none-any.whl (149kB)
   pipenv-pipenv-tests-1  | Installing collected packages: certifi
   pipenv-pipenv-tests-1  | Successfully installed certifi-2021.10.8
   pipenv-pipenv-tests-1  | Path: /root/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
   pipenv-pipenv-tests-1  | Installing Pipenv...
   pipenv-pipenv-tests-1  | Obtaining file:///pipenv
   pipenv-pipenv-tests-1  |     Complete output from command python setup.py egg_info:
   pipenv-pipenv-tests-1  |     Traceback (most recent call last):
   pipenv-pipenv-tests-1  |       File "<string>", line 1, in <module>
   pipenv-pipenv-tests-1  |       File "/pipenv/setup.py", line 55
   pipenv-pipenv-tests-1  |         print(f"\033[1m{s}\033[0m")
   pipenv-pipenv-tests-1  |                                  ^
   pipenv-pipenv-tests-1  |     SyntaxError: invalid syntax
   pipenv-pipenv-tests-1  |
   pipenv-pipenv-tests-1  |     ----------------------------------------
   pipenv-pipenv-tests-1  | Command "python setup.py egg_info" failed with error code 1 in /pipenv/
   pipenv-pipenv-tests-1 exited with code 1

   The docker image it relies upon
   (https://hub.docker.com/r/kennethreitz/pipenv-tests) hasn't been
   updated in 4 years, so I assume it's just not something people use
   anymore?

3. Relatedly, there was a Dockerfile at the root of this repo that
   appears to be unused. Let me know if it's used somewhere I'm not
   realizing, I can add it back!

4. https://kennethreitz.org/essays/be-cordial-or-be-on-your-way seems to
   be a broken link now. I found
   https://kennethreitz.org/essays/2013/01/27/be-cordial-or-be-on-your-way
   on Google.

5. ./run-tests.sh doesn't work for me. It's failing for the same
   reason described by @ncoghlan here:
   Cannot install into user site directory with editable source. pip#7953 (comment). He
   said something about a PIPENV_BOOTSTRAP environment variable, but I
   can't find any information about that.

[jfly]

Urg, that didn't quite work. I've added another commit that should hopefully do it.

[jfly]

@frostming, for the record, I'm happy with this PR and tests seem to be passing now.

[jfly]

Thanks, @frostming!