docs: add taskiq integration usage page

[lesnik512]

Adds the usage page for the new modern-di-taskiq integration
(modern-python/modern-di-taskiq),
plus its nav entries.

Changes

• docs/integrations/taskiq.md — usage page mirroring the other integration
  pages (install, setup_di, @broker.task + FromDI, sync-resolution /
  async-cleanup note, taskiq_message_provider for TaskiqMessage injection).
• mkdocs.yml — nav entry in both the top-level nav and the redirect section,
  ordered alphabetically among the framework integrations.

Verified with mkdocs build --strict (clean) and just lint-ci (clean).

⚠️ Do not merge until the package is published

The page instructs pip install modern-di-taskiq, which only works once the
package is on PyPI. Hold this PR until modern-di-taskiq has its first PyPI
release, so the live docs never reference an uninstallable package.

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