Add info about compiler.fileTimestamps and compiler.contextTimestamps

[jeffin143]

fix #4106

There was no good place found to write the info

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[jeffin143]

@chenxsan , We have compilation-hooks.md and compilation-object.md, but for compiler we just have compiler-hooks.md , may be we can pen down something for compiler.md too ? function and other details

[chenxsan]

Let's create a new page for those compiler objects.