User-focused docs: add a how-to for writing a new `Strategy`

[dotsdl]

Currently, our documentation for stratocaster only gives a brief description of what the library is and how to install it. We want to expand this documentation to give a potential user a how-to detailing how a user could write their own Strategy, similar to the how-to for writing gufe Protocols: https://gufe.openfree.energy/en/latest/how-tos/protocol.html