Document how to pass multiple files to Vale

This is a continuation of https://github.com/ansys/actions/issues/432, in particular of this comment:

```console
- name: "Install jq"
run: |
# Used to format Vale input as a JSON-formatted list of files
sudo apt install -y jq

- name: "Collect desired files"
run: |
# Find all .rst files excluding those in doc/source/api/
RST_FILES=$(find doc/source -type f -name "*.rst" ! -path "doc/source/api/*")

# Find all .py files inside the examples/ directory, excluding those with an underscore
PY_FILES=$(find examples -type f -name "*.py" ! -name "*_*")

# Combine both file lists and convert them to a JSON array
# TODO: include Python files too. See Vale issue
# https://github.com/errata-ai/vale/issues/858
# VALE_FILES=$(echo -e "$RST_FILES\n$PY_FILES" | jq -R . | jq -s .)
VALE_FILES=$(echo -e "$RST_FILES" | jq -R . | jq -s .)
echo "VALE_FILES=$(echo $VALE_FILES)" >> $GITHUB_ENV
```

Start by reading the continuation issue ansys/actions#432 and the referenced Vale issue #858, then inspect the repository's existing workflow documentation. Document how the shown jq and shell commands pass multiple .rst and .py files to Vale, including the exclusions, so a user can reproduce the workflow.

Written by the indexing model from the issue text.