Standardization pass through docs/ folder.

[ScottTodd]

• recommend an out-of-tree CMake build directory ../iree-build/ instead of in-tree build/
• consistently use .vmfb as VM module flatbuffer file extensions
• make some docs use CMake examples instead of Bazel examples
• prefix all shell commands with $
• indent shell commands two spaces when continuing onto multiple lines
• consistently use - for LLVM flags and -- for Abseil flags
• typo fixes

[GMNGeoffrey]

I so badly want copy-paste that strips the $. We should look at how that's implemented internally and see if we can easily add it.

[GMNGeoffrey]

Pretty sure @bjacob said this would create ODR violations and was bad

[ScottTodd]

We can follow up on that later, separate from this PR.

[GMNGeoffrey]

Yeah just noticed it :-)

[ScottTodd]

I so badly want copy-paste that strips the $. We should look at how that's implemented internally and see if we can easily add it.

Same... Discussed a bit on Discord. We have some options, none look super easy though.