Create nginx.org-build.yml

[eepifanova]

The reusable workflow to build nginx.org
Should be run on nginx/nginx.org

[oxpa]

LGTM

[eepifanova]

@thresheek plz review
I've added she for actions/checkout and moved the target to the repo var

[eepifanova]

@thresheek
Checked on the following workflow - completed successfully
https://docs.github.com/en/actions/sharing-automations/reusing-workflows#limitations

on:
  push:
    branches:
      - main
    paths:
      - 'binary/**'
      - 'css/**'
      - 'dtd/**'
      - 'img/**'
      - 'sources/**'
      - 'text/**'
      - 'tools/**'
      - 'xml/**'
      - 'xsls/**'
      - 'xslt/**'
      - 'yaml/**'
  
permissions:
  contents: read
  id-token: write

jobs:
  build:
    name: build site
    uses: nginx/ci-self-hosted/.github/workflows/nginx.org-build.yml@nginx.org        `