docs: add issue application guidance (Closes #98)

[vickman787]

Summary

Closes #98

Adds docs/ISSUE_APPLICATION_GUIDE.md: how to apply for an AnchorKit issue, example application comments, the assignment-before-work rule, PR linking expectations, and maintainer assignment notes, aligned with the existing auto-assign workflow. Links it from the README docs index and docs/CONTRIBUTOR_GUIDE.md.

Issue scope

Acceptance Criterion	Status	Implementation Evidence	Tests	Notes
Issue application guidance is added	Done	docs/ISSUE_APPLICATION_GUIDE.md (new)	N/A (docs)
Example contributor comments are included	Done	docs/ISSUE_APPLICATION_GUIDE.md § 3	N/A (docs)
Assignment-before-work rule is documented	Done	docs/ISSUE_APPLICATION_GUIDE.md § 4	N/A (docs)
PR linking expectations are documented	Done	docs/ISSUE_APPLICATION_GUIDE.md § 6	N/A (docs)
Maintainer assignment notes are included	Done	docs/ISSUE_APPLICATION_GUIDE.md § 8	N/A (docs)
README links to the guidance	Done	README.md docs index table	N/A (docs)

Affected packages / apps

• docs/
• Other (list):

Testing performed

Docs-only change; no code paths affected. Verified manually:

• All relative markdown links in the new doc and the two edited files resolve to existing files.
• Rendered the new doc locally to confirm table/list formatting and heading anchors match the table of contents.
  No automated test suite applies to this change.

Screenshots / recordings

N/A — no apps/web UI change.

Security impact

• This PR touches secret handling, callback URLs, network selection, or contract admin/authorization logic.
• No security-relevant surface touched.

Documentation impact

• Docs under /docs updated for this change.
• README updated (new doc added to the docs index).
• No user-facing behaviour changed — no docs update needed.

Maintainer Review Checklist (self-check)

This is a docs-only PR; only the PR self-check and Documentation sections of MAINTAINER_REVIEW_CHECKLIST.md Phase 1 apply. Stellar/Soroban/secret-handling/mainnet-safety sections are N/A — no source or contract code touched.

PR self-check

• References the original issue with Closes #NNN in the PR body.
• Acceptance criteria on the issue are checked off individually in the PR description.
• pnpm verify N/A — docs-only change, no source touched.
• pnpm check:boundaries N/A — no packages/*/src import changed.
• pnpm contract:test N/A — nothing under contracts/ changed.

Documentation

• Topic docs updated for new user-facing exports — new guide added.
• README docs index updated if new doc added.
• Security docs updated if new threat areas introduced — N/A, no new threat area.

Risk / follow-ups

None. This is additive documentation; no existing behaviour or workflow changes.