Add observables document

[rmouritzen-splunk]

Add observables document.

Add minor formatting tweaks to main Understanding OCSF markdown doc.

Reviewing the newly added observables documents as rendered markdown (annoyingly) requires navigating to the repo at the commit point, then finding the document. It took me a moment to figure this out, so I'll provide the steps here.

1. Click on "Commits" tab (between "Conversation" and "Checks").
2. On the row for the latest commit, look for and click the < > button on the right. The popup text says "Browse the repository at this point in the history".
3. Click on the Articles folder and then Defining and Using Observables.md.

Reviewing the changes to the existing Understanding OCSF.md document, the "Rich Diff" view helps, though viewing the fully rendered markdown shows a cleaner and more accurate view, though without diffs.