docs: Add bidirectional linking from sync to async methods

Complete bidirectional documentation linking by adding "See Also"
references from sync methods to their async counterparts.

This makes async alternatives immediately discoverable when users
are reading sync method documentation.

Changes:
- Server.has_session() → links to ahas_session()
- Server.new_session() → links to anew_session()
- Session.new_window() → links to anew_window()
- Session.rename_session() → links to arename_session()
- Window.kill() → links to akill()

Now both directions work:
- Async methods have always linked to sync versions
- Sync methods now link to async versions

This completes the documentation enhancement, providing perfect
discoverability in both directions for users working with either
sync or async APIs.

Author: tony

src/libtmux/server.py

@@ -805,6 +805,10 @@ def has_session(self, target_session: str, exact: bool = True) -> bool:
         Returns
         -------
         bool
+
+        See Also
+        --------
+        :meth:`ahas_session` : Async version of this method
         """
         session_check_name(target_session)
 
@@ -963,6 +967,10 @@ def new_session(
         ------
         :exc:`exc.BadSessionName`
 
+        See Also
+        --------
+        :meth:`anew_session` : Async version of this method
+
         Examples
         --------
         Sessions can be created without a session name (0.14.2+):

src/libtmux/session.py

@@ -964,6 +964,10 @@ def rename_session(self, new_name: str) -> Session:
         Raises
         ------
         :exc:`exc.BadSessionName`
+
+        See Also
+        --------
+        :meth:`arename_session` : Async version of this method
         """
         session_check_name(new_name)
 
@@ -1033,6 +1037,7 @@ def new_window(
 
         See Also
         --------
+        :meth:`anew_window` : Async version of this method
         :meth:`Window.new_window()`
 
         Examples

src/libtmux/window.py

@@ -757,6 +757,10 @@ def kill(
 
         ``$ tmux kill-window``.
 
+        See Also
+        --------
+        :meth:`akill` : Async version of this method
+
         Examples
         --------
         Kill a window: