docs: Add bidirectional cross-references between sync and async pane methods

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

Changes to src/libtmux/pane.py:

1. Pane.send_keys() → links to asend_keys()
   - Added "See Also" section after parameters, before examples

2. Pane.capture_pane() → links to acapture_pane()
   - Added "See Also" section after parameters

3. Pane.split() → links to asplit()
   - Added "See Also" section after parameters, before examples

This completes the bidirectional documentation linking pattern:
- Async methods already link to sync versions (added in previous commits)
- Sync methods now link to async versions (this commit)

Users can now easily discover async alternatives when reading sync
documentation and vice versa, providing seamless navigation between
the two API styles.

Related: Phase 1 async expansion - documentation improvements

Author: tony

src/libtmux/pane.py

@@ -838,6 +838,10 @@ def capture_pane(
             Negative numbers are lines in the history.
             `-` is the end of the visible pane
             Default: None
+
+        See Also
+        --------
+        :meth:`acapture_pane` : Async version of this method
         """
         cmd = ["capture-pane", "-p"]
         if start is not None:
@@ -873,6 +877,10 @@ def send_keys(
         literal : bool, optional
             Send keys literally, default False.
 
+        See Also
+        --------
+        :meth:`asend_keys` : Async version of this method
+
         Examples
         --------
         >>> pane = window.split(shell='sh')
@@ -1090,6 +1098,10 @@ def split(
         environment: dict, optional
             Environmental variables for new pane. tmux 3.0+ only. Passthrough to ``-e``.
 
+        See Also
+        --------
+        :meth:`asplit` : Async version of this method
+
         Examples
         --------
         >>> (pane.at_left, pane.at_right,