Docs: Update boa's main README.md

[nekevss]

This PR is to update the main README.md for boa.

Occasionally, we still receive issues from someone using the Boa crate, so this is mostly attempting to add some clarity to the main README for which crates boa-dev currently maintains as well as adding an initial walkthrough example for anyone who stops by the repo.

It changes the following:

• Adds Boa Crates section
• Adds Boa Engine Example
• Adds Documentation section for links to both the latest release and main.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	96,422	96,422	0
Passed	80,939	80,939	0
Ignored	3,567	3,567	0
Failed	11,916	11,916	0
Panics	0	0	0
Conformance	83.94%	83.94%	0.00%

[jedel1043]

Really liking the new README!

[HalidOdat]

I like the new README! Looks good to me! :)