[DOCS] Document the :lines: option of literalinclude

Add the `:lines:` option to the reference page: syntax, that overlapping
ranges are included once in the order of the file, that it is applied after
the markers and therefore counts within the marked region, and what happens
when the value selects nothing.

Each of these was rendered and read back before being written down.

Assisted-by: claude-code:claude-opus-5
Agent-Session: https://claude.ai/code/session_015QXXkquh2eQNBiTYA39Wss
Signed-off-by: Sebastian Mendel <sebastian.mendel@netresearch.de>

Author: CybotTM

docs/reference/restructuredtext/literalinclude.rst

@@ -47,6 +47,29 @@ publish the very content it was meant to exclude. The same happens when the mark
 when the ``end-before`` text occurs only above the line matched by ``start-after``. Rendering with ``--fail-on-log``
 turns any of these warnings into a failing build.
 
+Selecting lines by number
+=========================
+
+A region can also be selected by line number, which is useful for files that have no natural marker:
+
+..  code-block::
+
+    ..  literalinclude:: Example.php
+        :language: php
+        :lines: 1,3-5,20-
+
+Line numbers start at 1, ranges are inclusive, and an omitted end means "down to the last line". Overlapping ranges
+are included once, in the order of the file.
+
+``:lines:`` is applied *after* ``:start-after:`` and ``:end-before:``, so when they are combined the numbers count
+within the marked region rather than within the whole file.
+
+Prefer markers where a file has them: line numbers change with every edit above the selected region, and nothing in
+the source file records that the documentation depends on them.
+
+If the value cannot be read as a list of line numbers, or if it selects no line at all, a warning is logged and
+nothing is included.
+
 Options
 =======
 
@@ -62,6 +85,10 @@ Options
 ``:end-before:``
     Text of the line the included region ends before. The line itself is not included.
 
+``:lines:``
+    Line numbers to be included, for example ``1,3-5,20-``. Counted within the region selected by ``start-after``
+    and ``end-before``.
+
 ``:emphasize-lines:``
     Line numbers to be highlighted, for example ``3,5-6``. Counted within the included region.