feat: add workflow to validate build in submodules

[jn-av]

Summary

Currently, provider repos can push docs that break the build on the docs site. The workflow added
in this PR can be included in the submodules to test the build whenever changes to the docs happen, reducing the effort to fix things later

Workflow Example

The workflow on provider repos could look like this:

name: Test Docs Hub Build
on:
  pull_request:
    paths:
      - 'docs/**'
  push:
    branches:
      - main
    paths:
      - 'docs/**'
jobs:
  test-docs-hub-build-pr:
    if: github.event_name == 'pull_request'
    uses: SAP/crossplane-provider-docs/.github/workflows/test-submodule-build.yml@main
    with:
      provider-repo: ${{ github.event.pull_request.head.repo.full_name }}
      provider-ref: ${{ github.event.pull_request.head.sha }}
      submodule-path: docs/crossplane-provider-btp   # adjust per provider
  test-docs-hub-build-push:
    if: github.event_name == 'push'
    uses: SAP/crossplane-provider-docs/.github/workflows/test-submodule-build.yml@main
    with:
      provider-repo: ${{ github.repository }}
      provider-ref: ${{ github.sha }}
      submodule-path: docs/crossplane-provider-btp   # adjust per provider

This PR needs to be merged first to test the workflow out.