Intro URL orbs

[rosieyohannan]

Description

Add content to orbs intro to include URL orbs and introduce the differentiating term "registry orbs"

Reasons

A link to a GitHub and/or JIRA issue (if applicable).
Otherwise, a brief sentence about why you made these changes.

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs) 😸:

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Is there a "Next steps" section at the end of the page giving the reader a clear path to what to read next?
• Include relevant backlinks to other CircleCI docs/pages.

[briceicle]

nit: URL orbs don't strictly have to be stored in a git repository. Though we expect it to be the most common case.
Any publicly available content URL (e.g. a public S3 object) would also work (as long as the URL prefix is in the org's allow list)