document add-ons

[slivingston]

Create documentation about add-ons:

• mistyproxy
• vnc
• cam

Note that some tutorials already includes add-ons; e.g., tutorial_proxy_fixedmisty.md. However, dedicated pages about add-ons is more useful for API references and to understand add-ons in general, not only in application to specific deployments.

[slivingston]

This can be referred to from the hardshare documentation: https://docs.rerobots.net/hardshare/addons