Fix docs build failure caused by stale MkDocs theme override path

[Copilot]

The GitHub Actions build job for documentation was failing during mkdocs build because the site configuration referenced a theme override directory that no longer exists in the repository. This updates the docs config to match the current repository layout.

• Root cause

  • mkdocs.yml set theme.custom_dir to docs/overrides
  • docs/overrides is not present, causing MkDocs to abort with a configuration error

• Change

  • Removed the stale custom_dir entry from the Material theme configuration
  • Left the rest of the theme setup unchanged

• Impact

  • Restores successful documentation site builds in GitHub Actions
  • Avoids introducing placeholder directories or unused override scaffolding

theme:
  name: material
  logo: assets/nexus-logo-blue.png
  favicon: assets/nexus-logo-blue.png

[Copilot]

Pull request overview

Removes a stale MkDocs Material theme override configuration that referenced a non-existent directory, which was causing mkdocs build (and the GitHub Actions docs build) to fail.

Changes:

• Removed theme.custom_dir: docs/overrides from mkdocs.yml to eliminate the invalid path reference.

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