reproducible-builds: change/add titles in the "How" section

This is preparation work for the following commit.

(From yocto-docs rev: 0ff459c273704a281419a44ae5670068ac47f1b9)

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Reviewed-by: Guillaume Swaenepoel <guillaume.swaenepoel@smile.fr>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/test-manual/reproducible-builds.rst

@@ -122,9 +122,12 @@ The project's current reproducibility status can be seen at
 You can also check the reproducibility status on the Autobuilder:
 :yocto_ab:`/valkyrie/#/builders/reproducible`.
 
-===============================
-Can I test my layer or recipes?
-===============================
+===================================
+How can I test my layer or recipes?
+===================================
+
+With world build
+~~~~~~~~~~~~~~~~
 
 Once again, you can run a ``world`` test using the
 :ref:`oe-selftest <ref-manual/release-process:Testing and Quality Assurance>`
@@ -132,6 +135,9 @@ command provided above. This functionality is implemented
 in :oe_git:`meta/lib/oeqa/selftest/cases/reproducible.py
 </openembedded-core/tree/meta/lib/oeqa/selftest/cases/reproducible.py>`.
 
+Subclassing the test
+~~~~~~~~~~~~~~~~~~~~
+
 You could subclass the test and change ``targets`` to a different target.
 
 You may also change ``sstate_targets`` which would allow you to "pre-cache" some