build: Fix documentation generation in meson

libmbim uses an auxiliary tool to generate source, header and documentation section files. The section files are then used by gtkdoc to generate proper documentation. However, meson due to its inner workings, is not able to properly generate documentation because in the final section file generation the command does not finnish. The issue in meson is better explained here.

As a workaround in the meantime, the documentation must be generated prior installing libmbim. These can be done by building the libmbim-glib-doc target before running the install target:

ninja libmbim-glib-doc