message: docs: update links to point to data-plane-api repository
diff: This is the preferred style of adding dependencies that use Bazel for their
  build process.
  -1. Specify name and version in [external documentation](../docs/install/requirements.rst).
  +1. Specify name and version in [external documentation](https://github.com/envoyproxy/data-plane-api/blob/master/docs/root/install/requirements.rst).
  2. Define a new Bazel repository in [`bazel/repositories.bzl`](repositories.bzl),
  in the `envoy_dependencies()` function.
  5. Reference your new external dependency in some `envoy_cc_library` via the
  @@ -20,7 +20,7 @@ The shell script is executed by Bash, with a few Bazel-specific extensions.
  See the [Bazel docs for "genrule"](https://docs.bazel.build/versions/master/be/general.html#genrule)
  for details on Bazel's shell extensions.
  -1. Specify name and version in [external documentation](../docs/install/requirements.rst).
  +1. Specify name and version in [external documentation](https://github.com/envoyproxy/data-plane-api/blob/master/docs/root/install/requirements.rst).
  2. Add a BUILD file in [`bazel/external/`](external/), using a `genrule` target
  to build the dependency. Please do not add BUILD logic that replaces the
  dependency's upstream build tooling.
  @@ -43,7 +43,7 @@ an example.
  This is the older style of adding dependencies. It uses shell scripts to build
  and install dependencies into a shared directory prefix.
  -1. Specify name and version in [external documentation](../docs/install/requirements.rst).
  +1. Specify name and version in [external documentation](https://github.com/envoyproxy/data-plane-api/blob/master/docs/root/install/requirements.rst).
  2. Add a build recipe X in [`ci/build_container/build_recipes`](../ci/build_container/build_recipes)
  for developer-local and CI external dependency build flows.
  3. Add a build target Y in [`ci/prebuilt/BUILD`](../ci/prebuilt/BUILD) to consume the headers and
  @@ -55,6 +55,6 @@ and install dependencies into a shared directory prefix.
  # Updating an external dependency version
  -1. Specify the new version in [external documentation](../docs/install/requirements.rst).
  +1. Specify the new version in [external documentation](https://github.com/envoyproxy/data-plane-api/blob/master/docs/root/install/requirements.rst).
  2. Update the build recipe in [`ci/build_container/build_recipes`](../ci/build_container/build_recipes).
  3. `bazel test //test/...`
  