Add new vale rule for GLFM

What does this MR do?

After reviewing a few developer doc MRs about GLFM (ex: !107772 (merged)), I felt the usage was clear and we didn't need to explicitly spell out GitLab Flavored Markdown every time in these docs (or raise diff warnings about them). It's still important to be sure the regular product documentation is clear though, so I don't want to remove the rule completely.

This MR splits the vale rule for GLFM and GFM into it's own rule, which can then be excluded from development docs where the usage is clear (they introduce the terminology in the page with GitLab Flavored Markdown (GLFM) and GitHub Flavored Markdown (GFM)).

Before these changes, we have 127 warnings for GFFM and GFM across all docs. After we split this out and exclude it from these two development docs, there are only 11 warnings total, so we're removing 116 warnings.

Related issues

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