docs: add transport security troubleshooting guide

[pranjalbhatia710]

Summary

• adds a v2 docs page for DNS rebinding / transport security configuration
• explains the 421 Invalid Host header and 403 Invalid Origin header cases
• documents allowed hosts/origins, wildcard ports, and when disabling protection is appropriate

Closes #1798

Test plan

• uv run --frozen mkdocs build --strict
• git diff --check

[pranjalbhatia710]

Rebased this on current main and resolved the docs-nav conflict from the V2 documentation restructure. The transport-security page is now listed under the existing Advanced section instead of reintroducing the older top-level Documentation nav group.

Validation after rebase:

uv run --frozen mkdocs build --strict
# Documentation built successfully

git diff --check origin/main...HEAD
# no output

Pushed via SSH because the HTTPS token still lacks workflow scope and GitHub rejects pushes whose rebased history includes workflow-file changes from upstream.

[pranjalbhatia710]

Rebased this on current main again after the docs nav was restructured into the new topical layout. The transport-security page is now kept as a focused top-level docs page next to the existing Troubleshooting entry, without restoring the older Advanced subsections that moved elsewhere.

Validation after rebase:

uv run --frozen mkdocs build --strict
# Documentation built successfully

git diff --check origin/main...HEAD
# no output

Pushed via SSH because the HTTPS token still lacks workflow scope and GitHub rejects this rebased branch history while it contains upstream workflow changes.