Remove documentation pages for hosted/online projects

[abhro]

It becomes confusing to host the full documentation copy for packages such as AstroLib.jl at https://juliaastro.org/dev/modules/AstroLib when the package already has its own documentation website at https://juliaastro.org/AstroLib.jl/stable/. This sort of nesting duplicates unnecessary information and makes the site hard to navigate. For projects that already have their own documentation websites (like FITSIO.jl and Photometry.jl, but NOT AstroAngles.jl), we can just include a link to the package's own documentation website.

[abhro]

docs/make.jl states

# This make file compiles the documentation for the JuliaAstro website.
# It consists of the usual documeter structure, but also follows the approach
# of the SciML docs page https://github.com/SciML/SciMLDocs/blob/main/docs/make.jl
# by generating nested documentation for packages under the JuliaAstro organization.
# That way, docs for all packages are browesable and searchable in one place!

but the newer version of SciML docs (https://docs.sciml.ai) only have the standalone documents on there, with a menubar linking to all the packages. Making the menubar right now might be too much work, but adding a "package index" page might also work.

[abhro]

Also, in install.jl, the variable requiredmods is looked for in a given docs/pages.jl of a package. When looking at a package on its own, it's not immediately obvious why pages.jl would hold requiredmods, unless the requirement by this project is known. (Made the mistake with AstroImages.jl)