cravex2-reachability: Create documentation for reachability analysis

[pombredanne]

Publish user documentation and guide for reachability analysis

[ziadhany]

this is done,

Updated and/or created documentation files:

• https://github.com/aboutcode-org/scancode.io/blob/main/docs/images/tutorial-web-ui-analyze-symbol-reachability.png
• https://github.com/aboutcode-org/scancode.io/blob/main/docs/built-in-pipelines.rst
• https://github.com/aboutcode-org/scancode.io/blob/main/docs/index.rst
• https://github.com/aboutcode-org/scancode.io/blob/main/docs/scanpipe-pipes.rst
• https://github.com/aboutcode-org/scancode.io/blob/main/docs/tutorial_analyze_symbols_reachability.rst

see:

• Create documentation for reachability analysis #2211