Add missing documentation links

[glpuga]

Proposed changes

Adds missing documentation links:

• BeamSensorModel in Sensors page
• WeightedStateEstimator in StateEstimation page

Adds a section with a link to the Doxygen documentation from the Beluga README.md (before the only link was in the landing page of the repository).

Also fixes the path to the document generation script in GETTING_STARTED.md

Type of change

• 🐛 Bugfix (change which fixes an issue)
• 🚀 Feature (change which adds functionality)
• 📚 Documentation (change which fixes or extends documentation)

Checklist

Put an x in the boxes that apply. This is simply a reminder of what we will require before merging your code.

• Lint and unit tests (if any) pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• All commmits have been signed for DCO

[nahueespinosa]

LGTM!