Documentation: Document how to call Scorecard as a library.

[jeffmendoza]

• Enhance the go pkg documentation (ex: https://pkg.go.dev/github.com/ossf/scorecard/v5@v5.0.0/pkg/scorecard#Run) with description of how to run Scorecard with examples
• Add a section to the main README under Using Scorecard with a brief blurb about running as a library and linking to the go docs.
• (Possible / Later) Add a doc under the docs/ directory with a longer description of running Scorecard as a library

TBD: As we add more docs to scorecard.dev, where will this show up?

Source info:
One example here: #4254 (comment)
Design issue for new entrypoint: #3717

[jeffmendoza]

I'm starting work on the first item.