System docs

Add show system-docs command to generate system documentation.

• Implemented --format option allowing users to choose between: md (Markdown) and rst (ReStructuredText)

• Add all board connections documentation (if tx-board and rx-board are not specified) or filtered documentation (if tx-board and rx-board are specified)

• Generate documentation for all board connections: show generate system-docs --format rst

• Generate documentation for a specific board connection show generate system-docs --format md --tx-board serenity3 --rx-board serenity2