[Docs][Doc][C++] run-end-encoding functions not listed in compute functions

### Describe the bug, including details regarding any error messages, version, and platform.

The [C++ compute docs](https://github.com/apache/arrow/blob/ee6ff0a1b6b1bceedcea868265b4b0965d0056d0/docs/source/cpp/compute.rst#L4) do not list the `run_end_encode` and `run_end_decode` functions.

### Component(s)

C++, Documentation

Open docs/source/cpp/compute.rst at the compute functions section and compare its list with the available functions named in the issue. Add run_end_encode and run_end_decode to the documentation, then verify that both functions are listed clearly alongside the other compute functions.

Written by the indexing model from the issue text.