docs(examples): add regex-allowlist credential-leak example + index-page link

[chad-loder]

Summary

• Adds a new worked example, Avoid regex hostname-allowlist credential leaks (docs/examples/avoid-regex-hostname-allowlist-vulns.md), explaining the textbook URL-allowlist regex bug and the URLPattern fix.
• Adds a sly hyperlink on the docs home page (docs/index.md) pointing security-curious readers at the seed invoke-ai/InvokeAI#7518.
• Wires the new page into the Examples index (docs/examples/index.md — new "Security" section) and the site nav (properdocs.yml).

Why this example matters

invoke-ai/InvokeAI#7518 is an open issue on a ~26k★ Python AI project that surfaces a concrete credential-leak scenario:

remote_api_tokens:
  - url_regex: 'private.example'
    token: 'secret'

The intent reads cleanly — "attach secret when calling private.example." But re.search finds the regex source anywhere in the URL string, and a URL is not a flat character sequence, so two attacker-controlled URL shapes also match:

1. https://malicious.example/private.example/theft.safetensors  ·  path-segment fallthrough
2. https://private.example.malicious.example/theft.safetensors  ·  subdomain shadowing

The example walks through both attack shapes, the component-aware URLPattern fix, and the "host or any subdomain" variant via {:subdomain.}*private.example.

What this PR changes

File	Change
docs/examples/avoid-regex-hostname-allowlist-vulns.md	new — full worked example
docs/examples/index.md	adds Security section pointing at the new page
properdocs.yml	nav entry under Examples (alongside the webhook-shape validator)
docs/index.md	one extra clause closing the "differentiator" paragraph: [hand-rolled regexes for URL allowlists are routinely error-prone](...) linking to the InvokeAI issue

Test plan

• just docs — strict-mode build green (no broken links, nav consistent)
• just lint — all tools green (ruff, mypy, pyright, ty, semgrep, shellcheck, rumdl, codespell, interrogate, validate-pyproject)
• Signed commit (ED25519)

No code changes; docs only.