Update _DEBUG macro reference

[Rageking8]

• Mention /LDd as it also defines _DEBUG, as per:

  cpp-docs/docs/preprocessor/predefined-macros.md

  Line 168 in 0ffedaa

  - `_DEBUG` Defined as 1 when the [`/LDd`](../build/reference/md-mt-ld-use-run-time-library.md), [`/MDd`](../build/reference/md-mt-ld-use-run-time-library.md), or [`/MTd`](../build/reference/md-mt-ld-use-run-time-library.md) compiler option is set. Otherwise, undefined.

  cpp-docs/docs/build/reference/md-mt-ld-use-run-time-library.md

  Line 29 in 0ffedaa

  |**/LDd**|Create a debug DLL. Defines `_MT` and `_DEBUG`.|

• Add link to "Predefined macros" in "See also" section

• Simplify and add some links

• Update metadata

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[v-regandowner]

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