📝 docs: add SEP-0010 explain protocol

[zrr1999]

Proposal summary

• Add seps/SEP-0010-compiler-as-documentation.md as a standalone Standards Track SEP for compiler-owned explain surfaces, concept docs, and diagnostic teaching metadata.
• Reconnect the SEP suite around that split by adding minimal cross-references from README.md, GLOSSARY.md, SEP-0005, SEP-0006, and seps-index.json.
• Keep this stacked PR narrowly scoped to the design document and SEP cross-links; concrete schema/catalog rollout can follow separately once the design boundary settles.

Linked discussion

Canonical discussion thread: #46

This PR is intentionally stacked on top of #45:

• base PR: 📝 docs: refine vision, SEP process, and signature surface #45

Checklist

• I opened or linked a public Discussion or Issue for the pitch before submitting this draft SEP.
• I linked the canonical discussion thread.
• I used the correct SEP template for this proposal type.
• I updated front matter metadata and required sections.
• I considered the SEP-0000 guiding questions in the relevant proposal sections.

Notes for reviewers

Review focus

1. Does SEP-0010 stay clearly in the teaching/explain layer rather than drifting back into SEP-0006 compiler-core responsibilities?
2. Are the SEP-0005 and SEP-0006 forward references to SEP-0010 at the right boundary?
3. Is it reasonable to land this PR as a doc-and-cross-reference slice first, leaving concrete schema/catalog artifacts for a later follow-up if needed?

Validation

• prek run -a: passed