ref-manual: add "Mixin" term

Also add a target (and use) a target
for the "Long Term Support Release" section.

(From yocto-docs rev: 98d6f2066fcdb20e63b8da6ed52d0f3590b15195)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/release-process.rst

@@ -87,6 +87,8 @@ stable release.
    exception to this policy occurs when there is a strong reason such as
    the fix happens to also be the preferred upstream approach.
 
+.. _ref-long-term-support-releases:
+
 Long Term Support Releases
 ==========================
 
@@ -120,16 +122,16 @@ of stable and :term:`LTS` releases.
 
    In some circumstances, a layer can be created by the community in order to
    add a specific feature or support a new version of some package for an :term:`LTS`
-   release.  This is called a "mixin" layer. These are thin and specific
+   release.  This is called a :term:`Mixin` layer. These are thin and specific
    purpose layers which can be stacked with an :term:`LTS` release to "mix" a specific
    feature into that build. These are created on an as-needed basis and
    maintained by the people who need them.
 
    Policies on testing these layers depend on how widespread their usage is and
-   determined on a case-by-case basis. You can find some "mixin" layers in the
+   determined on a case-by-case basis. You can find some :term:`Mixin` layers in the
    :yocto_git:`meta-lts-mixins </meta-lts-mixins>` repository. While the Yocto
    Project provides hosting for those repositories, it does not provides
-   testing on them. Other "mixin" layers may be released elsewhere by the wider
+   testing on them. Other :term:`Mixin` layers may be released elsewhere by the wider
    community.
 
 Testing and Quality Assurance