docs: draft Ticking.md

[nfrisby]

Adds a document that discusses ticking and how its used within the Consensus Layer.

[nfrisby]

Rendered: https://github.com/IntersectMBO/ouroboros-consensus/blob/nfrisby/doc-ticking/docs/website/contents/for-developers/Ticking.md

[geo2a]

Thanks @nfrisby for this write-up, it's a wealth of information! I've left a couple of minor suggestions and a question, but I think the document is ready to be merged as-is.