✨ Feature: Add watch/hot-reload functionality

[matt-goldman]

Running blake serve does not currently provide any kind of hot reload capability. This would be great for live viewing content changes.

Proposed approach:

• Currently blake serve just calls blake bake then dotnet run. Could change to dotnet watch.
• Pair with above, create watcher for markdown files and re-run blake bake when the change. Note that if we do this we should ensure ✨ Feature: Incremental builds #41 is completed first.
• In theory, running blake bake will generate new Razor files, and dotnet watch will take care of the rest from there.

Tasks:

• Evaluate the feasibility of this approach
• Consider whether other approaches are valid and/or better
• Devise implementation plan

Important: The final point indicates that the goal at this stage is a plan. First deliverable is a backlog - create sub-issues indicating the work that will be completed. Each sub-issue should be incrementally deployable (i.e., it should be ok to ship 1, then do 2, etc). Each sub-issue should include tests.