docs(install): start writes the config, so stop telling people to seed it first

The page opened the last step with a mkdir and a cp of .env.example, then
start. That is backwards: start creates the config dir and writes COLLIE_MUX
into .env itself, after asking which multiplexer to mirror (cli/mux.ts). The
copy was work the reader did not owe, and read in the printed order it is
worse than redundant — run that cp after a start and it lands the example file
on top of the answer start just wrote.

Now: start first, and seeding the file is the alternative for someone who
wants to decide up front, marked as something to do BEFORE the first start.