Add FAQ regarding theme overrides

Currently, the documentation refers to creating new themes a lot.
It is much less apparent how one can customize an existing theme by
overriding individual templates or adding new ones. This commit adds
an FAQ for this, and also mentions the mechanism in the FAQ on custom
templates.

docs/faq.rst

@@ -64,6 +64,19 @@ How do I create my own theme?
 
 Please refer to :ref:`theming-pelican`.
 
+Can I override individual templates without forking the whole theme?
+====================================================================
+
+Yes, you can override existing templates of the theme that you are using, or
+add new templates, via the ``THEME_TEMPLATES_OVERRIDES`` variable. For example,
+to override the page template, you can define the location for your templates
+like this::
+
+    THEME_TEMPLATES_OVERRIDES = ["templates"]
+
+You can then define a custom template in ``templates/page.html``.
+See :ref:`settings/themes` for details.
+
 I want to use Markdown, but I got an error.
 ===========================================
 
@@ -122,7 +135,8 @@ For content in Markdown format::
     Template: template_name
 
 Then just make sure your theme contains the relevant template file (e.g.
-``template_name.html``).
+``template_name.html``). If you just want to add a new custom template to an
+existing theme, you can also provide it in a directory specified by ``THEME_TEMPLATES_OVERRIDES`` (see :ref:`settings/themes`).
 
 How can I override the generated URL of a specific page or article?
 ===================================================================