Remove extra User Guide readthedocs TOC entries by turning extra H1 headings into H2 (#6986)

readthedocs treats H1 headings as document titles in the sidebar table of contents.
This change turns all extra H1 headings into H2 in the "Metal-Specific Functionalities" doc.
These extra H1 headings seem to be errors, given that they are inconsistent, and the H2s after them do not belong. For examples, the heading "Resource Types" is H2, but "Array Types" is H1, and "Mesh Shader Support" is H1, but the following "Header Inclusions and Namespace" is not a subtopic of the Mesh Shader Support.

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