Document how to put webdev behind a proxy server

See https://stackoverflow.com/questions/55230404/dart-webdev-server-requires-index-html

We should check that it's possible to have a proxy in front of webdev and still get all the debugging and rebuilding stuff, then write up a document on how to set up a typical server.

Start with the linked Stack Overflow report and verify whether webdev works behind a proxy while preserving debugging and rebuilding. Document a typical proxy-server setup and the required webdev configuration, with completion defined by a tested setup that retains both behaviors.

Written by the indexing model from the issue text.