Refactor documentation

[nettle]

Why:
The documentation was one 500 line page, no structure.
The contribution page mixed reporting issues with the developer setup.

What:

• Split docs/README.md into pages
• Support static rendering with Docsify 5
• Add pages for the public API, troubleshooting and contributing
• Reduce CONTRIBUTING.md to reporting issues and a pointer to the docs
• Update the contents to correspond current status

Addresses:
#156

[furtib]

Looks great!
I was unable to open the HTML file locally, I needed to start a webserver to be able to view it!
Are we / Should we create a github pages for this html page, so users can view this more easily?

[nettle]

I was unable to open the HTML file locally, I needed to start a webserver to be able to view it!

Hm... that should work as a static page, no webserver is required.
I'll check...

Are we / Should we create a github pages for this html page, so users can view this more easily?

Yes, exactly, the simplest way: just publish docs/ as https://ericsson.github.io/rules_codechecker/
(old example: https://nettle.github.io/documentation-template)

[nettle]

I was unable to open the HTML file locally, I needed to start a webserver to be able to view it!

Hm... that should work as a static page, no webserver is required. I'll check...

Modern web browsers block loading resources (mostly scripts) when opening local HTML files considering those as potential security threats:

Unsafe attempt to load URL file:///.../index.html from frame with URL file://.../index.html.
'file:' URLs are treated as unique security origins.

So, yes, unfortunately we cannot see this as local file