Finish Block 8 cross-links and RU status synchronization

[Lay007]

Follow-up to #43 after the new OFDM/CSS labs were added.

Already completed in #43:

• Lab 8.10, 8.20 and 8.21 are present in RU and EN mkdocs.yml navigation;
• the navigation checker and repository pytest contract pass;
• all three Russian lab pages are fully localized;
• the compact lab index and English top-level status page include the new track.

Remaining tasks

• Add direct links to Lab 8.10, 8.20 and 8.21 from the Block 8 RU/EN overview and workflow pages.
• Update docs/ru/status.md so its Block 8 row matches docs/status.md.
• Review whether the existing QPSK Lab 8.8/8.9 material should receive dedicated RU/EN MkDocs wrapper pages and navigation entries.

Acceptance criteria

• mkdocs build --strict passes.
• Block 8 overview pages expose the OFDM and CSS learning route without requiring learners to search the navigation tree.
• RU and EN status descriptions no longer disagree about Block 8 readiness.
• Any remaining numbering gap is intentional and documented.