docs(label): Add guidelines for preview labels.

[edonehoo]

Closes #4829

docs links (these are all one after the other):

• https://pf-org--pr-4855-site.surge.sh/components/label/design-guidelines#beta-and-preview-labels
• https://pf-org--pr-4855-site.surge.sh/components/label/design-guidelines#deprecated-labels
• https://pf-org--pr-4855-site.surge.sh/components/label/design-guidelines#quick-start-labels

Note that, to align with these guidelines, we might want to make these org updates too (can handle in a follow-up if this makes sense):

• Update our “Beta” tag to be purple to align with preview labels in products. OR, determine if/why Beta should be blue. In PF, purple=info, which can carry over to nonstatus purple maybe
• Remove “Demo” label from pf.org: It doesn’t offer meaningful value and page content makes it clear when there are demos. If we want it to stay, maybe just re-color it, but I would opt for removing

[patternfly-build]

Preview: https://pf-org--pr-4855-site.surge.sh

[edonehoo]

@andrew-ronaldson can you take a look and lmk if you think these recommendations look okay from a design perspective--especially with your product knowledge? Any different direction you think makes more sense?