Anatomy of SURB + anatomy of SPHINX packets doc pages

[mfahampshire]

Is your issue either:

• [ ] a fix to existing documentation (e.g. fixing a broken link or incorrect command)
• [ ] an enhancement (e.g. adding a description for an undocumented feature)

Please briefly describe your issue:

complete in parallel to redrawing diagrams with mermaid / nomnoml

riff on 'anatomy of stark' series title

• diagrams +
• explainer of packet use in network +
• link to qus raised in research chan +
• further reading