message: docs: Update build instructions for docs
  Partial fix for
diff: -# Developer-local docs build
  +# Building documentation locally
  +
  +There are two methods to build the documentation, described below.
  +
  +In both cases, the generated output can be found in `generated/docs`.
  +
  +## Building in an existing Envoy development environment
  +
  +If you have an [existing Envoy development environment](https://github.com/envoyproxy/envoy/tree/master/bazel#quick-start-bazel-build-for-developers), you should have the necessary dependencies and requirements and be able to build the documentation directly.
  ```bash
  ./docs/build.sh
  ```
  -The output can be found in `generated/docs`. By default configuration examples are going to be validated during build.
  -To disable validation, set `SPHINX_SKIP_CONFIG_VALIDATION` environment variable to `true`:
  +By default configuration examples are going to be validated during build. To disable validation,
  +set `SPHINX_SKIP_CONFIG_VALIDATION` environment variable to `true`:
  ```bash
  SPHINX_SKIP_CONFIG_VALIDATION=true docs/build.sh
  ```
  +## Using the Docker build container to build the documentation
  +
  +If you *do not* have an existing development environment, you may wish to use the Docker build
  +image that is used in continuous integration.
  +
  +This can be done as follows:
  +
  +```
  +./ci/run_envoy_docker.sh 'docs/build.sh'
  +```
  +
  +To use this method you will need a minimum of 4-5GB of disk space available to accommodate the build image.
  +
  +# Creating a Pull Request with documentation changes
  +
  +When you create a Pull Request the documentation is rendered by CircleCI.
  +
  +If you are logged in to CircleCI (it is possible to authenticate using your Github account), you can view
  +the rendered changes.
  +
  +To do this:
  +- click `Details` in the `ci/circleci: docs` check at the bottom of the Pull Request.
  +- click `ARTIFACTS` in the CircleCI dashboard
  +- browse to the documentation root at `generated/docs/index.html`.
  # How the Envoy website and docs are updated
  