feat: adding docs for CAPX v1.9

[abhay-nutanix]

Add CAPX v1.9.x documentation

Adds documentation for CAPX v1.9.x (now the latest release), based on the
existing v1.8.x docs. This PR includes the new version's doc set, wires it
into site navigation, and updates the latest alias.

Changes

• New docs/capx/v1.9.x/ doc set – full set of pages cloned from v1.8.x
  and updated for the new release.
• Navigation – added a v1.9.x (latest) section in mkdocs.yml mirroring
  the v1.8.x structure, and dropped the (latest) label from v1.8.x.
• latest alias – repointed the docs/capx/latest symlink from v1.8.x
  to v1.9.x.

What's different from v1.8.x

• Validated Integrations – updated support matrices:
  • CAPI: v1.9.x supports CAPI v1.11.x, v1.12.x, v1.13.x (drops
    v1.9.x/v1.10.x). Matrix extended out to CAPI v1.13.x.
  • AOS: v1.9.x is validated against AOS 7.5 only (newer AOS baseline;
    columns shifted to 6.8 → 7.5).
  • Prism Central: v1.9.x is validated against pc.7.5 only (columns
    shifted to pc.2023.x → pc.7.5).
  • AOS 7.3 cloud-init/cloud-config note updated to reference v1.9.x.
• User Requirements – added the Update Virtual Machine Custom Attributes
  permission to both the base permission list and the upgrade permission list.
  Upgrade section now covers v1.8.x → v1.9.x.
• Upgrade Procedure – new tasks/capx_v19x_upgrade_procedure.md (replaces
  capx_v18x_upgrade_procedure.md); final step upgrades CAPX to v1.9.0.
• Getting Started – upgrade-procedure link/label updated to v1.9.x.

Testing

• mkdocs build passes with no broken-link or missing-nav warnings for the
  new v1.9.x pages.