docs: Rewrite a solid chunk of the documentation.

[asiekierka]

• Split out two additional sections from Technical information:
  • Usage, which has content that is additionally of use to homebrew end users,
  • Additional information, which contains documentation and design notes that are not of use to most homebrew end users or developers.
• Add an Optimization guide, which documents some ways of optimizing homebrew.
• Add an Upgrade guide, which documents breaking changes in BlocksDS.
• Add a section to the Migrating from devkitARM document which covers updating legacy homebrew.
• Rewrite the Design choices document to be more accurate to the current state of BlocksDS.
• Rewrite the C/C++ support document to be more accurate to the current state of BlocksDS and duplicate less information.
• Other minor fixes.

[AntonioND]

Thanks! The changes are now live here: https://blocksds.github.io/docs/