DOC: Update documentation for gravitational wave module

[Lucia-Fonseca]

Description

This pull request updates the documentation for the gravitational wave module. It closes #357

Checklist

• [x] Follow the Contributor Guidelines
• ~[ ] Write unit tests~
• [x] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[Lucia-Fonseca]

Agreed. Thank you for suggesting this. What would be an appropriate scale value? @itrharrison do you have any suggestion?

Since the purpose here is to demonstrate b_band_merger_rate I would simplify the rest of the config file and just sample luminosities using skypy.utils.schechter and the appropriate scale parameter.

[rrjbca]

Should be able to work it out from the source code for scheter_lf_magnitude and luminosity_from_absolute_magnitude probably something like luminosity_from_absolute_magnitude(m_star, 'Lsun_B') for some value e.g. m_star = -21 @ntessore have I got that right?