doc: Split the "Substitutes" section into subsections.

* doc/guix.texi (Substitutes): Move information into sections, and create one
  new subsection regarding substitution failure.
  (Top): In the detailed listing, add entries for the new subsections.
  (Official Substitute Server): Move info about hydra here.  Add information
  that explains when substitutes are enabled by default.
  (Substitute Server Authorization): Move information about how to enable or
  disable substitutes here.
  (Substitute Authentication): Move information about how Guix authenticates
  substitutes here.
  (Proxy Settings): Move information about how to download substitutes via
  proxy here.
  (Substitution Failure): New section.  Add information about how Guix handles
  substitution failure.  Be extra clear about behavior concerning --fallback.
  (On Trusting Binaries): Move information about trusting binaries here.
  (Invoking guix-daemon): Remove an incorrect statement about when substitutes
  are enabled by default.
  (Common Build Options): Add cross-references for the --fallback option.

Co-authored-by: George Clemmer <myglc2@gmail.com>

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