docs: stop recommending meson compile

docs: stop recommending meson compile

With meson-0.60, meson compile stopped working with some targets:

$ meson compile -C build update-man-rules
ERROR: Can't invoke target `update-man-rules`: ambiguous name. Add target type and/or path: `PATH/NAME:TYPE`

This is obviously a regression in meson, but based on a chat with the
maintainers, it seems that there's some disagreement as to whether 'meson
compile' is useful and how exactly it should work. Since we're already at
meson 0.60.3 and this hasn't been fixed, and people generally don't seem to
consider this an issue, let's return to documenting the usual practice of
'ninja -C build' that just works everywhere.

(Since nobody has raised any fuss in systemd, it means that people are
generally using the shorter form during development too. I only noticed
because I pasted a command from the release docs when preparing -rc1.)