integrations: split into scms/ and scanners/ subcategories

Restructures the flat /integrations/<x> layout into two clean
subcategories that match the way the integrations actually divide:
SCM platforms (where Pixee delivers fixes) and scanning tools (where
findings come from). Moves URLs from /integrations/<x> to
/integrations/{scms,scanners}/<x> and adds redirects.

SCMs (4 pages) under docs/integrations/scms/:
- github.md       (renamed from github-platform.md, content unchanged)
- gitlab.md       (split out from scm-platform-reference.md)
- azure-devops.md (split out from scm-platform-reference.md)
- bitbucket.md    (split out from scm-platform-reference.md)

Scanners (14 pages) under docs/integrations/scanners/:
- appscan.md, checkmarx.md, codeql.md, contrast.md, gitlab-sast.md,
  semgrep.md, snyk-code.md, sonarqube.md, veracode.md (moved from flat)
- polaris.md, fortify.md   (split out from commercial-scanners.md)
- trivy.md, defectdojo.md  (split out from oss-aggregator-scanners.md)
- gitlab-sca.md            (newly authored to match the SCA scope —
  this content was missing from PR #117 and needs colleague review)

Removes three consolidated wrapper pages now that each scanner / SCM
has its own page: commercial-scanners.md, oss-aggregator-scanners.md,
scm-platform-reference.md.

Sidebar (autogen, no hand-built):
- /integrations/overview        (sidebar_position: 1)
- /integrations/sarif-universal (sidebar_position: 2)
- Source Control subcategory    (position: 3, generated-index landing)
- Scanning Tools subcategory    (position: 4, generated-index landing)

Each subcategory gets a generated-index landing at /category/source-control
and /category/scanning-tools respectively, which renders a card list of the
pages inside.

Redirects (added to docusaurus.config.js):
- /integrations/<flat-scanner>      -> /integrations/scanners/<x>  (9 rules)
- /integrations/github              -> /integrations/scms/github
- /integrations/{commercial-scanners,oss-aggregator-scanners,scm-platforms}
                                    -> /integrations/overview
- Pre-existing /code-scanning-tools/* and /integrations/sonar redirects
  retargeted to the new /integrations/scanners/<x> URLs.

Body content: 2 internal links updated from /integrations/codeql to
/integrations/scanners/codeql in the new github.md page.

Overview rewrite: integrations-overview.md updated to reflect the new
two-category structure, refreshed coverage matrix (13 scanners), and
new SCM links pointing at /integrations/scms/<x>.

Migration archive: migration/integrations_restructure.py captures the
mechanical operations (file moves, frontmatter updates, body-link
fixup, wrapper deletions) for posterity. Will not be re-run.

Verification:
- yarn build clean (77 docs processed; was 72 before this commit).
- yarn serve verified all 4 SCM pages, all 14 scanner pages, both
  generated-index landings, 8 sample redirects, and sidebar order.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dhafley