Docs: Make QDOC_INDEX_DIR optional

[ferdnyc]

If the Qt documentation index files are not available (which they may not be, since some distro installs of Qt6 don't package the documentation), the GammaRay manual can still be built. All of its links to the system docs will be broken, but that doesn't render it unusable.

Allow building the documentation even without access to the .index files for the system docs, but print a message about broken links.

Fixes #867

qdoc will complain mightily, but it will still generate usable documentation.

.../docs/manual/examples/qt3d-geometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/examples/qt3d-geometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/examples/quick-batching.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/examples/quick-batching.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/examples/quick-batching.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/examples/quick-event-handling.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/gammaray-client.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/gammaray-object-inspection.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/gammaray-object-inspection.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-object-inspection.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/gammaray-object-inspection.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3d-inspector.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3d-inspector.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3d-inspector.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3dgeometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3dgeometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3dgeometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3dgeometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3dgeometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qt3dgeometry.qdoc:14: (qdoc) warning: Can't link to 'Qt 3D'
.../docs/manual/gammaray-qtquick2-inspector.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'
.../docs/manual/gammaray-qtquick2-inspector.qdoc:14: (qdoc) warning: Can't link to 'Qt Quick'

[CLAassistant]

All committers have signed the CLA.

[ferdnyc]

So, it seems like the CLA bot can't handle more than one active check at a time (per-user?). I just submitted two PRs close together (~5 minutes apart). The first one passed the CLA check with no problems. But the second was coming up unsigned. It only sorted itself out after I let all of the jobs on first PR complete, then force-pushed this branch to re-trigger the CI jobs.

(It's possible a force-push would've been successful sooner than that, since the CLA bot finishes its own work pretty quickly. OTOH, for that same reason it would've been finished with its work on the other PR long before I opened this one, so... ¯\_(ツ)_/¯

[ferdnyc]

Oh! Or maybe it's that there was a commit generated by pre-commit here, before I squashed and force-pushed to get rid of it? Perhaps I prematurely closed my #814 and should reopen it, if that's the case.