Doxygen docs improvements

[jlblancoc]

Do a better work describing SLAM methods in:

• [ ] mrpt::slam::CMetricMapBuilderICP
• [ ] mrpt::slam::CMetricMapBuilderRBPF

Better description of range-only localization and mapping:

• [x] New centralized webpage about range-only localization and SLAM.
• [ ] pf-localization
• [ ] rbpf-slam
• [ ] CLandmarksMap

Metric maps:

• [x] Write a page with all types of observations, metric maps, and what combinations are implemented for insertion and likelihood evaluation.

Navigation:

• [x] Enumerate all score names / variables in the new rnav system.
• [ ] Write docs for the rnav pipeline with graphs, explaining all signals (callbacks), etc.

Config ("INI") files:

• [x] Explain format, preprocessor, comments, etc.

General:

• [x] Provide doc pages for each module.

[jlblancoc]

Done maps-observations table: http://reference.mrpt.org/devel/maps_observations.html