docs: sphinixfy gcov.txt and move to dev-tools

No textual changes beyond formatting.

Cc: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/tools.rst

@@ -17,3 +17,4 @@ whole; patches welcome!
    coccinelle
    sparse
    kcov
+   gcov

MAINTAINERS

@@ -5118,7 +5118,7 @@ GCOV BASED KERNEL PROFILING
 M:	Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
 S:	Maintained
 F:	kernel/gcov/
-F:	Documentation/gcov.txt
+F:	Documentation/dev-tools/gcov.rst
 
 GDT SCSI DISK ARRAY CONTROLLER DRIVER
 M:	Achim Leubner <achim_leubner@adaptec.com>