docs: add per-module pages, README table, and small src cleanup

[nolte]

Summary

Expand the documentation surface for the reusable Taskfile includes: replace
the flat README bullet list with a module table, add per-module pages under
docs/modules/, wire them into the mkdocs navigation, and fix two small
correctness issues in the k8s and mkdocs source includes.

Changes

• Replace the bullet list in README.md with a table that lists each module's
  tasks and key tunable variables.
• Add per-module documentation pages under docs/modules/ for mkdocs, kind,
  pre-commit, and k8s, each with a Tasks / Variables / Example layout.
• Add a nav: section to mkdocs.yml exposing an Overview plus a Modules
  group, so the new pages are reachable from the rendered site.
• Fix kubectl create namespace argocd | true → || true in
  src/taskfile-include-k8s.yaml so a pre-existing namespace no longer aborts
  the bootstrap.
• Normalise indentation under bootstrap.cmds in the same file.
• Drop the stray YAML document marker (---) at the top of
  src/taskfile-include-mkdocs.yaml.

Linked issues

None

Testing

• pre-commit run --all-files — all hooks pass (check-yaml, end-of-file-fixer,
  trailing-whitespace).
• Manually reviewed the rendered diff of README.md and the new docs/modules/
  pages.

Risk / rollout notes

Low risk. Documentation-only on the consumer-visible surface; the two src/
changes are bug fixes (| true → || true makes re-runs idempotent; removing
the stray --- has no semantic effect on Taskfile parsing).