Document how to install prerequisites in examples READMEs

[chrisinmtown]

Add server url /openapi to JWT example
Add TODO to SQLAlchemy example, does not work with current versions
Correct name of module in v3 instructions

Fixes #1948, #1987

[chrisinmtown]

Please review this draft, if you like this approach I will extend to other README.rst files.

If it's not obvious, I'm trying to figure out how to work with you all. Baby steps :)

[RobbeSneyders]

Thanks @chrisinmtown, this looks good to me. Would be great to extend it to the other examples.

[chrisinmtown]

I am extending the changes to all the examples. Please check example sqlalchemy which has become a bit stale. I dropped the convert_unicode parameter in the create_engine function and then it started, but the first GET attempt yields this:

sqlite3.ProgrammingError: SQLite objects created in a thread can only be used in that same thread. The object was created in thread id 123145410666496 and this is thread id 140704480263680.

Do you have time for this?

[RobbeSneyders]

Let's leave that example as is for now then. Would be great if you could add a notice that it is outdated to the README.

For the other examples, the flask extra is not needed for most of them.

[chrisinmtown]

Let's leave that example as is for now then. Would be great if you could add a notice that it is outdated to the README.

I will ask the cloud a few questions and see if I can figure it out :)

For the other examples, the flask extra is not needed for most of them.

I see that Flask is not needed for enforcedefaults, frameworks and hello_async. I copy-pasted those a little too quickly, will adjust.

[chrisinmtown]

This PR has primarily documentation changes. I tweaked one API spec and one requirements file to make the examples work. Please consider.