A Github Action for deploying a WordPress plugin to the WordPress.org plugin repository with quite a few comments explaining each piece.

deploy-to-wp-org.yml

# The name of the Github Action that displays in github.com/<username>/<repo>/actions
name: Deploy to WordPress.org Repository

# Here we can define the events "on" which the action should be triggered.
on:

  # Since we want to publish new versions of our plugin, we only want this action to
  # run when publishing a new release.
  #
  # The released version of the plugin will then be deployed to the repository.
  #
  # This allows us to run and manage plugin releases from a single location.
  release:

    # run only when a new release is published, but not when it's classified as a pre-release.
    types: [released]

# A list of jobs involved in this workflow.
jobs:

  # A unique job identifier.
  #
  # Github Actions can have multiple jobs and each can be referenced by its name.
  # However, we only need to run a few steps here and they can be handled in a single job.
  deploy_to_wp_repository:

    # The proper name for the job being run.
    name: Deploy to WP.org

    # The environment this job should run on. In the context of WordPress, ubuntu-latest is
    # pretty typical. Since we are only interacting with git and subversion, Ubuntu is perfect
    # for this.
    #
    # Github does offer other platforms if you need them: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idruns-on
    runs-on: ubuntu-latest

    # Every job has a specific set of steps that it goes through to do its "work".
    #
    # Each step has a two key elements:
    # • Name
    # • a "run" command (an arbitrary CLI command to execute) OR a "uses" command that pulls in and executes a 3rd party action.
    steps:

    # Most workflows begin by checking out the repository into the workflow filesystem.
    #
    # This is just like cloning a repository except it only checks out the specific commit
    # the job is executed for. In our case here, the commit that the release is attached to.
    - name: Checkout code
      uses: actions/checkout@v2
    
    # Optional: If your plugin is using composer dependencies, we want to include them
    # WITHOUT the dev dependencies.
    - name: Build
      run: composer install -o --no-dev

    # Using 10up's deployment action, we deploy our plugin to the WordPress repository.
    # You can view that action here: https://github.com/10up/action-wordpress-plugin-deploy
    - name: WordPress Plugin Deploy
      
      # You can add unique ids to specific steps if you want to reference their output later in the workflow.
      #
      # Here, this unique identifier lets us use the output from the action to get the zip-path later.
      id: deploy

      # The use statement lets us pull in the work done by 10up to deploy the plugin to the WordPress repository.
      uses: 10up/action-wordpress-plugin-deploy@stable

      # Steps can also provide arguments, so this configures 10up's action to also generate a zip file. 
      with:
        generate-zip: true
      
      # Steps can also set environment variables which can be configured in the Github settings for the
      # repository. Here, we are using action secrets SVN_USERNAME, SVN_PASSWORD, and PLUGIN_SLUG which
      # authenticate with WordPress and lets the action deploy our plugin to the repository.
      #
      # To learn more about setting and using secrets with Github Actions, check out: https://docs.github.com/en/actions/security-guides/encrypted-secrets?tool=webui#about-encrypted-secrets
      env:
        SVN_USERNAME: ${{ secrets.SVN_USERNAME }}
        SVN_PASSWORD: ${{ secrets.SVN_PASSWORD }}
        SLUG: ${{ secrets.PLUGIN_SLUG }}

    # After the deploy, we also want to create a zip and upload it to the release on Github. We don't want
    # users to have to go to the repository to find our plugin :).
    - name: Upload release asset
      uses: actions/upload-release-asset@v1
      env:
        # Note, this is an exception to action secrets: GH_TOKEN is always available and provides access to
        # the current repository this action runs in.
        GITHUB_TOKEN: ${{ secrets.GH_TOKEN }}

      with:
        # Get the URL for uploading assets to the current release.
        upload_url: ${{ github.event.release.upload_url }}

        # Provide the path to the file generated in the previous step using the output.
        asset_path: ${{ steps.deploy.outputs.zip-path }}

        # Provide what the file should be named when attached to the release (plugin-name.zip)
        asset_name: ${{ github.event.repository.name }}.zip

        # Provide the file type.
        asset_content_type: application/zip