Fill out the various sections with content

[DilumAluthge]

These sections are currently empty and need content to be added:

• "Documentation"
• "Proposing Changes"
• "Code"
• "Walkthrough"
• "Core Developers"

We should not start advertising this guide broadly until these empty sections have been filled out.

[inkydragon]

So we're splitting devdocs into "devguide" outside of the manual for guidance on how to develop Julia and "Internals" in the manual for documentation about the internals of a specific version of Julia.

So we need to move some of Julia's documents to here.

I guess devdoc just needs the following parts:

• "Developing/debugging Julia's C code".
• "Building Julia"

https://github.com/JuliaLang/julia/blob/dfca90fcfec3e9519ea015580586dfaeae8aef03/doc/make.jl#L236-L251

By using git-filter-repo we can move files and keep the commit history.
ref: JuliaMath/SpecialFunctions.jl#409 (comment)