Go into more detail on the two non-legacy build modes.
I decided use create a dedicated document documention the build process.
`index.rst` only keeps the "Getting Started" part. `README.rst` is
vastly outdated with the still valid parts integrated into the new
documentation. `SupportAndFAQ.rst` is unstructured and keeps only the
non-building parts.
The the new building document is written in Meltdown, following the
decision from LLVM.
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