Dynamic config docs refresh

[rosieyohannan]

Description

Improvements to main dynamic config overview page https://internal.circleci.com/docs/dynamic-config/:

• added info to explain main concepts
• structure improvements
• increase the scope of use cases

Improvements to the dynamic config how-to guide https://internal.circleci.com/docs/using-dynamic-configuration/:

• Fixed incorrect description of path filtering example - I think the snippet got updated in line with an orb update at some point but the description didn't
• Added more accompanying explainers for the snippets
• Add two new examples: Config splitting, and config splitting plus packing and validating a shared config file

To be added in a separate PR: Diagrams to show various setups, make a sample project in circleci-public to show examples building

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.

[imlogang]

I fixed the 4 linting errors in #8784