Enable community contributions to ESDM (doc PRs, shared resources)

[goloroden]

What is this task about?

A user wants to contribute to ESDM – not code, but resources: skills, language packs, and documentation improvements. The building blocks are partly in place and partly missing:

• The esdm repository is public and the documentation lives in it (documentation/docs/), so documentation PRs are already possible. What is missing is a CONTRIBUTING.md that invites and explains them.
• There is no home for shared community resources (skills, language packs). An "awesome" list was floated but carries the usual risk of going stale.

This task is to decide how external contributions to ESDM should work and to put the entry points in place. Scope is ESDM for now.

What needs to be done to complete this task?

• Add a CONTRIBUTING.md that invites documentation PRs and explains how to contribute (quick win, since doc PRs already work on the public repo).
• Decide whether and how to host shared community resources (skills, language packs): a dedicated repository, a curated list, or another form – and how to keep it from going stale.
• Decide how contributed resources are reviewed and maintained.
• Point to the relevant resource types from the docs.

What else should we know?

• Origin: raised by a user who wants to contribute resources (not code) and noticed there is no clear entry point.
• Related to Explore "language packs" for consistent vocabulary in generated artifacts #11 (language packs) and Decide how agent skills use ESDM (product specs, tests, fixtures) #12 (agent skills) – both are exactly the kind of shared resource this would host.
• Out of scope: a community discussion channel (the former Discord) is handled separately; extending contribution/hosting to ESDB or the wider family (ESDM first).
• Non-goal: about enabling contributions, not about a specific resource.