DOC: Update documentation for gravitational wave module#359

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Lucia-Fonseca wants to merge 2 commits intoskypyproject:mainfrom

Lucia-Fonseca:gw_doc

Commits on Oct 29, 2020

Add plot for three mergers'
Lucia-Fonseca
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Commits on Nov 3, 2020

Adding table to config file and fix text
Lucia-Fonseca
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