Minor clarifications to global.conf page

[chausner-audeering]

These changes aim to reduce some potential confusion about the global.conf documentation (see https://github.com/conan-io/conan/issues/9529#issuecomment-940884107). It adds an example global.conf file and tries to make it more clear that the snippet shown under "Tools configurations" is profile syntax, not global.conf syntax.

[prince-chrismc]

@czoido Is this a good spot for adding an example?

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:x: Chris Mc
:x: chausner-audeering

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