Convert examples/* to proper cargo-manifest structure.

[maaku]

I hope this is well-received as I didn't ask before authoring. While evaluating floem for use in my own app, I started with the examples and got frustrated that I couldn't build them / run them / doc them from the workspace directory with the standard cargo run --example <name> command, or to build/test them all with e.g. cargo build --examples. This is because they are created as workspace crates, not as proper cargo examples.

This PR converts them all to standard cargo manifest examples.

[panekj]

You should run them as cargo run -p <name> also I'd prefer if they were kept as they are now, since it's not an uncommon way to store/run examples

[timsueberkrueb]

Whether or not the structural changes will be merged, I think the added doc comments are helpful.

[jrmoulton]

I'm going to go ahead and close this for now as I think the current structure is preferred but thanks for taking the time to work on improving things!