docs(internals): add SparseArray doc page

why: SparseArray internal API had no documentation page.
what:
- Create docs/internals/sparse_array.md with automodule directive
- Add sparse_array to docs/internals/index.md toctree

Author: tony

docs/internals/index.md

@@ -13,6 +13,7 @@ dataclasses
 query_list
 waiter
 constants
+sparse_array
 ```
 
 ## Environmental variables

docs/internals/sparse_array.md

@@ -0,0 +1,14 @@
+# Internal Sparse Array - `libtmux._internal.sparse_array`
+
+:::{warning}
+Be careful with these! Internal APIs are **not** covered by version policies. They can break or be removed between minor versions!
+
+If you need an internal API stabilized please [file an issue](https://github.com/tmux-python/libtmux/issues).
+:::
+
+```{eval-rst}
+.. automodule:: libtmux._internal.sparse_array
+   :members:
+   :undoc-members:
+   :show-inheritance:
+```