DIP-287 Add inReplyTo to Reply Notes

[wesbiggs]

Problem

The current lack of specificity in Reply Note content could lead to certain corner cases that may be difficult or nondeterministic for applications to handle.

Link to GitHub Issue(s): #287

Solution

Add requirement for inReplyTo with DSNP Content URI in Activity Content Note document.
This ensures that a given Reply Note can only ever be in reply to a single parent in its thread.

Change summary:

• Updated Spec Versions (pre-1.4.0 for Activity Content and DSNP)
• Changes to Note specification
• Additional examples

Steps to Verify:

1. Documentation-only change. Follow normal documentation generation steps.