WIP - Adding an allowed-values documentation page

[Rene2mt]

DO NOT MERGE. THIS PR IS WORK IN PROGRESS (WIP)

This WIP PR adds a separate documentation page for allowed values (issue #11). The added markdown page was created by:

1. Running the following OSCAL-CLI command to get a comprehensive list of all allowed-values (builtin OSCAL as well as in our developed external constraints):

oscal-cli list-allowed-values results/fedramp-allowed-values-list.yaml -c constraints/fedramp-external-allowed-values.xml

2. Then used yq to adjust the generated YAML

yq '.locations.[].constraints[] |= pick(["location", "target", "values", "allow-other", "source"])' results/fr-allowed-values.yaml > results/fr-allowed-values-transformed.yaml

3. Then, using yaml-to-markdown to convert the yaml list of allowed values to a Markdown page.

Step 2 needs more work to filter / select only keys of interest for documentation. Step 3 has some formatting issues (e.g. wide tables). Also many of the allowed-values constraints do not have identifiers, so the table columns / headers are inconsistent.

GOAL 1 - By having having all the allowed-values documentation in one place (similar to registry), it should be easier for FedRAMP OSCAL content authors to find the allowed-values guidance they might be looking for. It also simplifies documentation on other pages, as we can have documentation pages (like https://automate.fedramp.gov/documentation/ssp/4-ssp-template-to-oscal-mapping/) reference (with deep links) the allowed-value page content, and do away with the current blue callout boxes that we have now.

GOAL 2 - A secondary goal would be to generate the allowed-values page automatically so that as we add new or modify any allowed-values constraints in Metaschema, the documentation is automatically updated as well.