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.)

docs/TRANSLATORS.md

@@ -27,7 +27,7 @@ To create a translation to a language not yet available, start by creating the
 initial template:
 
 ```
-$ meson compile -C build/ systemd-pot
+$ ninja -C build/ systemd-pot
 ```
 
 This will generate file `po/systemd.pot` in the source tree.
@@ -51,7 +51,7 @@ using the `poedit` GUI editor.)
 Start by updating the `*.po` files from the latest template:
 
 ```
-$ meson compile -C build/ systemd-update-po
+$ ninja -C build/ systemd-update-po
 ```
 
 This will touch all the `*.po` files, so you'll want to pay attention when
@@ -75,7 +75,7 @@ using `git checkout -- po/` after you commit the changes you do want to keep.)
 You can recompile the `*.po` files using the following command:
 
 ```
-$ meson compile -C build/ systemd-gmo
+$ ninja -C build/ systemd-gmo
 ```
 
 The resulting files will be saved in the `build/po/` directory.