docs: add contribution directions roadmap

[qazW12345]

Summary

Adds docs/CONTRIBUTION-DIRECTIONS.md as a non-normative roadmap for building more connected, reusable mathematics across the LeanFrontier corpus.

This is the upstreamed and refreshed version of the contribution-directions dossier discussed in #186.

What changed

• moves the dossier into docs/, as requested in Upstreaming the contribution-directions dossier, and build errors in receiver reports #186;
• removes fork-specific PR states, contributor-specific sequencing, and references to private/fork workflow;
• updates the corpus map to current upstream main;
• treats LeanFrontier.Geometry.FordCircleTangency as accepted infrastructure, so the Ford–Farey–Descartes direction is now immediately actionable;
• treats LeanFrontier.MarkovTree.jump_descends_ordered_positive as accepted infrastructure and updates the Markov/Farey direction around the remaining tree/path representation gap;
• preserves the ranked A–K research directions;
• preserves the important negative/boundary guidance, including:
  • the Calkin–Wilf vs Stern–Brocot ordering correction;
  • the warning not to conflate the two unrelated reflect operations;
  • the Markov/Farey boundary around the Markov uniqueness conjecture;
  • the recommendation not to create graph-padding bridges merely for import connectivity.

Scope

This is documentation only. It does not modify the submission contract, receiver, policy, workflows, or mathematical source.

The document explicitly states that CONTRACT.md and the live submitter guidance remain authoritative, and that contributor-specific branch state / PR queues should not be encoded in the shared roadmap.

Closes the contribution-directions part of #186.

[carlok]

Thanks! Superseded by #200, which carries this commit unchanged (your authorship) so it can pass validate: maintenance/ PRs from forks are validated as ordinary submissions. Details in #186.