doc: fix references to mailing lists

The mailing list hosting software change introduced some changes to
where mailing lists are hosted.  Update the docs to point at the
specific sublists rather than the top-level site.  (See issue zephyrproject-rtos#7437).

Signed-off-by: David B. Kinder <[email protected]>

README.rst

@@ -88,11 +88,12 @@ support systems:
   Security related issue tracking is done in JIRA.  The location of this JIRA
   is https://zephyrprojectsec.atlassian.net.
 
-* **Mailing List**: The `Zephyr mailing lists`_ are perhaps the most convenient
+* **Mailing List**: The `Zephyr Development mailing list`_ is perhaps the most convenient
   way to track developer discussions and to ask your own support questions to
-  the Zephyr project community.  Notice there are specific `Zephyr mailing list
-  subgroups`_ for announcements and developers.
-  You can read through message archives to follow
+  the Zephyr project community.  There are also specific `Zephyr mailing list
+  subgroups`_ for announcements, builds, marketing, and Technical
+  Steering Committe notes, for example.
+  You can read through the message archives to follow
   past posts and discussions, a good thing to do to discover more about the
   Zephyr project.
 
@@ -107,7 +108,7 @@ support systems:
 .. _Getting Started Guide: http://docs.zephyrproject.org/getting_started/getting_started.html
 .. _Contribution Guide: http://docs.zephyrproject.org/contribute/contribute_guidelines.html
 .. _Zephyr GitHub wiki: https://github.com/zephyrproject-rtos/zephyr/wiki
-.. _Zephyr mailing lists: https://lists.zephyrproject.org/
+.. _Zephyr Development mailing list: https://lists.zephyrproject.org/g/devel
 .. _Zephyr mailing list subgroups: https://lists.zephyrproject.org/g/main/subgroups
 .. _Sample and Demo Code Examples: http://docs.zephyrproject.org/samples/samples.html
 .. _Security Overview: http://docs.zephyrproject.org/security/security-overview.html