IngestArch: Add introductory content

[karenzone]

Ingest architecture documentation consists mostly of architecture diagrams, use cases, caveats, and so forth. Some introductory content is still needed to help users form a mental model and to help them establish an understanding to build on.

Related: #18

[karenzone]

ON HOLD: Deferred to prioritize individual arch topics

[mergify]

This pull request does not have a backport label. Could you fix it @karenzone? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[mergify]

This pull request is now in conflict. Could you fix it @karenzone? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch <remote-repo>
git checkout -b add-content <remote-repo>/add-content
git merge <remote-repo>/main
git push <remote-repo> add-content

[karenzone]

This content lives on in the latest Ingest overview #1373