MINOR: [Docs] local_timestamp kernel docs are not linked in python docs (#39274)

### Rationale for this change

local_timestamp kernel docs are linked in [cpp](https://arrow.apache.org/docs/cpp/compute.html#timezone-handling) but not in [python docs](https://arrow.apache.org/docs/python/api/compute.html#timezone-handling).

### What changes are included in this PR?

This adds a rst link in python docs

### Are these changes tested?

No

### Are there any user-facing changes?

Change will be visible in the docs

Authored-by: Rok Mihevc <rok@mihevc.org>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

Author: rok

docs/source/python/api/compute.rst

@@ -468,6 +468,7 @@ Timezone Handling
    :toctree: ../generated/
 
    assume_timezone
+   local_timestamp
 
 Associative Transforms
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