Turn on GitHub pages deployment

[abhro]

The package has a fully functioning docs site, and it would be worthwhile to serve from the gh-pages branch for an easily accessible docs site.

There is a copy on base juliaastro.org, but that also includes a lot of other packages and tightly integrates all astro-related packages (or at least their docs). See issue JuliaAstro/JuliaAstro.github.io#30

[giordano]

Done: https://juliaastro.org/AstroAngles.jl/dev/

[abhro]

Thank you!!