Docs: make URLs not linkable (#11553)

These URLs are just examples to communicate how the URLs would be for
subprojects, but they don't exist. I'm marking them as code, so links are not
generated for them.

Closes #11358

docs/user/subprojects.rst

@@ -15,8 +15,8 @@ This is useful for:
 
 For a main project ``example-project``, a subproject ``example-project-plugin`` can be made available as follows:
 
-* Main project: https://example-project.readthedocs.io/en/latest/
-* Subproject: https://example-project.readthedocs.io/projects/plugin/en/latest/
+* Main project: ``https://example-project.readthedocs.io/en/latest/``
+* Subproject: ``https://example-project.readthedocs.io/projects/plugin/en/latest/``
 
 .. seealso::
 
@@ -38,8 +38,8 @@ If the example project ``example-project`` was set up with a custom domain,
 ``docs.example.com``, the URLs for projects ``example-project`` and ``example-project-plugin`` with alias ``plugin`` would
 respectively be at:
 
-* ``example-project``: https://docs.example.com/en/latest/
-* ``example-project-plugin``: https://docs.example.com/projects/plugin/en/latest/
+* ``example-project``: ``https://docs.example.com/en/latest/``
+* ``example-project-plugin``: ``https://docs.example.com/projects/plugin/en/latest/``
 
 Using aliases
 -------------