docs: fix strict build + update all docs for v7.0.0

[TexasCoding]

Summary

Fixes the failing Docs workflow (mkdocs build --strict, broken since the v6.0.0 merge) and brings all docs current for v7.0.0. Docs-only — no code, no wheel impact.

The build failure

mkdocs build --strict aborted on a broken link in docs/migration.md:

• [changelog](../CHANGELOG.md) — a cross-tree link mkdocs can't resolve (CHANGELOG lives at the repo root, not under docs/). Switched to the absolute GitHub URL, matching the working pattern already used elsewhere in the file.
• Also fixed a broken in-page anchor #v5--v600 → #v5-v600.

v7.0.0 accuracy

• New v6 → v7.0.0 migration section — price_cents→price, announcements soft-deprecation, the 3 new endpoints, and the additive field/param additions, with a migration snippet.
• subaccounts — transfer_position price_cents → price (Decimal dollars) in the quick-ref table, prose, and example; netting example fixed to iterate netting_configs (+ show the new exchange_index); removed a stale netting_enabled_subaccounts reference.
• exchange — deprecation admonition on announcements(); corrected the example to the actual model fields (type/message/delivery_time/status, not the non-existent title).
• pagination — noted announcements() is deprecated / 404s.
• portfolio — documented the new balance(*, exchange_index=...) param.
• perps — noted the two new klear margin methods (active_obligations, settlement_estimate_by_asset_class).
• OpenAPI version bumped 3.23.0 → 3.24.0 in README, docs/index.md, and CLAUDE.md (historical "when added" refs left intact).

Test plan

• uv run --group docs mkdocs build --strict — builds clean (was aborting)
• All code examples verified against the actual v7.0.0 model/method signatures
• Markdown-only; ruff / mypy / tests unaffected

This should land before tagging v7.0.0 so the release commit's docs are accurate and the Docs site rebuilds green.

[claude]

Review

Docs-only PR that fixes the mkdocs build --strict failure (bad cross-tree CHANGELOG link, bad in-page anchor) and brings the docs current for v7.0.0. I cross-checked every code-facing claim in the diff (method signatures, param names/types, endpoint paths, model fields, deprecation behavior, and runnable example code) against the actual source under kalshi/, and everything checks out precisely — transfer_position(price=Decimal(...)), exchange.announcements() deprecation + Announcement fields, klear.margin.active_obligations() / settlement_estimate_by_asset_class(), communications.quotes.get_for_rfq(), portfolio.balance(exchange_index=...), and SubaccountNettingConfig.exchange_index/netting_configs all match the code exactly. The anchor fix (#v5--v600 → #v5-v600) is also correct given the toc extension's default slugify with no custom config in mkdocs.yml.

One small accuracy gap:

README.md:14, docs/index.md:6, CLAUDE.md:125 — operation count still says "101 operations" despite this PR documenting a genuinely new main-API operation.

docs/migration.md's new "v6 → v7.0.0" section (and CHANGELOG.md) lists communications.quotes.get_for_rfq() as a new endpoint added in this same 3.23.0 → 3.24.0 spec sync. CommunicationsResource is wired into the main KalshiClient/AsyncKalshiClient (not PerpsClient/KlearClient), so it counts toward the "101 operations across 19 resources" figure quoted in README.md, docs/index.md, and CLAUDE.md — that figure appears to need bumping to 102 alongside the v3.23.0 → v3.24.0 version bump this PR already makes in those same three files. (The other two new endpoints, klear.margin.active_obligations() and settlement_estimate_by_asset_class(), are correctly excluded since they fall under the separately-counted Klear "9 operations" figure.)

Everything else — the broken-link fix, the anchor fix, the new migration section, and all the per-resource doc updates — looks accurate and ready to land. Nice, thorough work verifying examples against actual signatures.