Add warnings and documentation about `radec` singularity/warping for high declination angles

[ortk95]

From #323

• [x] Add singularity issue to common issues documentation page.
• [x] Add warning to documentation (e.g. plot_wireframe_radec),
• [x] Point to alternatives (e.g. plot_wireframe_km) once #324 completed
• [ ] Possibly add automated warning when creating radec plots at high declination angles