reorganizing the GC config doc

[Maoni0]

I rearranged sections in this doc - the only text I changed beyond that is in the "Manage resource usage for Server GC", I changed

For more information about some of these settings,

to

For more information about the first 3 settings,

this splits the "Manage resource usage" section into 2 - general and specific to Server GC. and moved settings under them correctly.

also moves all other sections to be before the 2 "Manage resource usage" sections.

created a top level section called "LOH specific settings" for the 2 LOH specific settings.

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docs/core/runtime-config/garbage-collector.md	Runtime configuration options for garbage collection