docs: fix the strict-mode Pages build

[singaraiona]

What & why

The v2.8.0 release job failed at the Pages step with Aborted with 10 warnings in strict mode!. (The red mkdocs-material "MkDocs 2.0" banner in the same log is a red herring.)

Reproduced locally against origin/dev: all ten warnings are dangling links out of docs/grouping-engine-scaling-results.md into bench/groupby_shapes/results/... and test/test_agg_contract.c — paths outside docs_dir, so mkdocs cannot resolve them. The five top-level engineering docs missing from nav were only INFO, not warnings, but they were orphan pages either way.

Removed

Raw measurement dumps belong with the benchmark harness, not the published site:

• aggregation-fast-path-results.md
• grouping-engine-scaling-results.md

The two references to the scaling results in grouping-engine-scaling-plan.md now point at bench/groupby_shapes/ instead of the deleted page.

Published

New "Design Notes" nav section, so the plans are reachable rather than orphaned:

• aggregation-fast-path-plan.md
• grouping-engine-scaling-plan.md
• multicast-implementation-plan.md

Also listed multicast-usage.md under the .mc namespace — it was an orphan page too.

Verification

mkdocs build --strict run locally with .github/requirements-docs.txt:

• origin/dev: Aborted with 10 warnings in strict mode! (exit 1)
• this branch: Documentation built in 3.19 seconds (exit 0)

Also checked: no remaining references to either removed doc in docs/ or mkdocs.yml, and every top-level docs/*.md is now in nav or not_in_nav.

Note

pages.yml only triggers on pushes to master, so PR CI never builds the docs — this class of failure can currently only surface during a release. Worth a separate PR-triggered strict-build job.