[docs] Add Getting Started chapter (migrate from README)

[bittner]

Bandit's documentation has a strong focus on explaining Bandit development topics. Bandit users are puzzled, because the documentation gives absolutely no hints about how to install, use and configure (the use of) Bandit. Luckily, the project README does that in detail.

Other F/LOSS projects prove that it's a good idea to keep the README short and crisp, and move (user-focused) instructions to the documentation. – This is exactly what this PR does. The README prominently links to the documentation, so all is good.

Resolves: #603

Refactoring Details

1. (Installation ... VC Integration) goes to "Getting Started" chapter.
2. (Configuration) is omitted as it replicates content in the "Configuration" chapter.
3. (Per Project CLA ... Exclusions) is added as section "Bandit Settings" to the "Configuration" chapter; the existing content gets "Scanning Behavior" as a section title.
4. (Vulnerability Tests ... Extending Bandit) is omitted: These sections are already included in the CONTRIBUTING document; the "Writing Tests" section is also part of the "Bandit Test Plugins" chapter.
5. (Under Which Version): This questions goes into a new chapter "FAQs".

I tried to change as little as possible of the original formatting (to keep the history of the content transition clean). I may update some of the formatting after this PR has been merged, or as additional commits here, if preferred.

[bittner]

I rebased the PR and resolved the conflict with the README. This could be merged now.

[bittner]

Can someone please approve the GHA workflow to be started and afterwards approve the changes, so that the PR could be merged?

[ericwb]

LGTM. The README is more concise now. I do think it could use some further work. The readme is mentioned how to contribute twice, yet no indication on how to install. References are typically at the end of a doc. I also think a gif demoing Bandit in action would be nice. Future PRs can address these things.

[bittner]

Thanks for merging! 👍

[bittner]

Building the docs fails because they're still built with Python 2.7 and there is a version conflict. This should be easy to fix by having the docs built by Python 3.

[bittner]

Looks like the Python version is to be set in the RTD project settings.

@lukehinds Can you set the "Python Interpreter" option to "CPython 3.x" and retrigger the build, please?

[ericwb]

@lukehinds also can you please add all the maintainers as admin to read-the-docs. Thanks!