Encourage placing preset includes near the beginning of a preset
file and ensure the example shows that usage. Move the prose
discussing includes to its own section to improve discoverability
and break up paragraphs to make each main point harder to miss.
Also clarify ${sourceDir} to remove any ambiguity with regard to
its meaning in included files.
Issue: #23214
Since commit 8bc5c8961e (CMakePresets.json: Add the ability to
conditionally disable presets, 2021-03-10, v3.21.0-rc1~464^2)
the example requires presets version 3 support, which is not
available until CMake 3.21. CMake 3.20.0 can't open v3 presets.
Make cmakeMinimumRequired compatible with the example's version.
