Update dh_auto documentation.

Update dh_auto documentation.

* dh_auto.pod -> dh_auto.7 - contains general information about dh_auto,
  its features, build systems supported by default (the latter is injected by
  man/dh_auto_pod script from Debian/Debhelper/Buildsystem/*.pm PODs via
  placeholders (#PLACEHOLDER#))
* POD in Debian/Debhelper/Buildsystem/*.pm -> dh_auto_<buildsystem>.7 - build
  system specific information.
* dh_auto_* -> dh_auto_*.1 - relatively shorty description of the specific
  dh_auto_* program and build system specific info for that step injected from
  Debian/Debhelper/Buildsystem/*.pm with man/dh_auto_pod script.
* man/dh_auto_pod $step - generates full dh_auto_$step POD (replaces
  placeholders).
* man/dh_auto_pod - generates full dh_auto.pod (replaces placeholders).

Signed-off-by: Modestas Vainius <modestas@vainius.eu>