Move GDB/MI commands related to support-query to their own @node.

A number of commands provide the capability to query the debugger
about support for various features, and one of them in particular
(-list-features), is expected to grow as new features get added.
-list-target-features should also grow a bit over time, but probably
slower.

These commands deserve their own section and @node.

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI): Add "GDB/MI Support Commands" entry
        in menu.
        (GDB/MI Variable Objects): Adjust reference to "-list-features"
        command, now in a new node.
        (GDB/MI Support Commands): New node, with its contents being
        extracted from the "GDB/MI Miscellaneous Commands" node.
        A small paragraph introducing the section is also added at
        the start.
        (GDB/MI Miscellaneous Commands): Delete the description of the
        -info-gdb-mi-command, -list-features and -list-target-features
        commands, now hosted in the "GDB/MI Support Commands" node.