Add improved documentation for V2021

[pombredanne]

This should consolidate anything that may be in the wiki and provide comprehensive usage documentation

• Update changelog
• Update README and add screenshots Improve README before v3 #982 Update README with new screenshot(s) of ScanCode workbench #811
• Update examples Improve examples  #1031
• Deprecate HTML app Deprecate html app and corresponding wiki documentation #1157
• Validate and fix ABOUT files Check and update thirdparty .ABOUT files for correctness, including in the CI #934 Missing top level ABOUT file #302
• publish doc web site

[steven-esser]

We have this currently https://github.com/nexB/scancode-toolkit/wiki/License-Policy-Plugin.

The plugin itself may need to be better documented.

[pombredanne]

This is really what @AyanSinhaMahapatra is working on.

[AyanSinhaMahapatra]

So, #1784 is the first step. Making sure tests for documentation builds are in place, with all requirements in thirdparty/.
Next, I'll test cross-sphinx links (To maintain links with nexB/aboutcode). Then comes the actual migration of the docs.