QASP: Add Accessibility standard

[jpmckinney]

The question to resolve is how to operationalize the policy, not whether to have a policy about accessibility as such.

From https://derisking-guide.18f.gov/qasp/:

Deliverable	Performance Standard(s)	Acceptable Quality Level	Method of Assessment
Accessibility	Web Content Accessibility Guidelines 2.1 AA standards	0 errors reported using an automated scanner, and 0 errors reported in manual testing	Pa11y

[jpmckinney]

Added Accessibility row to QASP.

I tested pa11y and it works.

One approach is to add a sitemap to public websites, which pa11y can use. pa11y is Node.js, so we'd either have to run it manually, or start up e.g. Django's runserver in CI, and then run pa11y against it.

I've configured CI for the data registry: open-contracting/data-registry#317

I added a page to the docs and opened issues in relevant repositories.