doc: Add "Sending a Patch Series" subsection.

Searching for `git send-email' took you to the end of `Submitting Patches'
section which said

   You may use your email client or the ‘git send-email’ command.

without further warning or remark.

* doc/contributing.texi (Sending a Patch Series): Move information about
debbugs bug 15361 to subsection.  Add git-send-email index entries.
(Submitting Patches): Reference it.

doc/contributing.texi

@@ -298,11 +298,7 @@ This mailing list is backed by a Debbugs instance accessible at
 of submissions.  Each message sent to that mailing list gets a new
 tracking number assigned; people can then follow up on the submission by
 sending email to @code{@var{NNN}@@debbugs.gnu.org}, where @var{NNN} is
-the tracking number.  When sending a patch series, please first send one
-message to @email{guix-patches@@gnu.org}, and then send subsequent
-patches to @email{@var{NNN}@@debbugs.gnu.org} to make sure they are kept
-together.  See @uref{https://debbugs.gnu.org/Advanced.html, the Debbugs
-documentation}, for more information.
+the tracking number (@pxref{Sending a Patch Series}).
 
 Please write commit logs in the ChangeLog format (@pxref{Change Logs,,,
 standards, GNU Coding Standards}); you can check the commit history for
@@ -434,7 +430,22 @@ Please follow our code formatting rules, possibly running the
 
 When posting a patch to the mailing list, use @samp{[PATCH] @dots{}} as
 a subject.  You may use your email client or the @command{git
-send-email} command.  We prefer to get patches in plain text messages,
-either inline or as MIME attachments.  You are advised to pay attention if
-your email client changes anything like line breaks or indentation which
-could potentially break the patches.
+send-email} command (@pxref{Sending a Patch Series}).  We prefer to get
+patches in plain text messages, either inline or as MIME attachments.
+You are advised to pay attention if your email client changes anything
+like line breaks or indentation which could potentially break the
+patches.
+
+@unnumberedsubsec Sending a Patch Series
+@anchor{Sending a Patch Series}
+@cindex patch series
+@cindex @code{git send-email}
+@cindex @code{git-send-email}
+
+When sending a patch series (e.g., using @code{git send-email}), please
+first send one message to @email{guix-patches@@gnu.org}, and then send
+subsequent patches to @email{@var{NNN}@@debbugs.gnu.org} to make sure
+they are kept together.  See
+@uref{https://debbugs.gnu.org/Advanced.html, the Debbugs documentation}
+for more information.
+@c Debbugs bug: https://debbugs.gnu.org/db/15/15361.html