GH-50735: [C++] Add run-end encoding functions to compute docs

[VedantRalekar]

Summary

Add the run_end_encode and run_end_decode functions to the C++ compute documentation.

Changes

• Add run_end_decode to the Structural transforms table.
• Add run_end_encode to the Structural transforms table.
• Add the corresponding documentation for run_end_encode.

Related Issue

Fixes #50735

• GitHub Issue: [Docs][Doc][C++] run-end-encoding functions not listed in compute functions #50735

[github-actions]

⚠️ GitHub issue #50735 has been automatically assigned in GitHub to PR creator.

[VedantRalekar]

Hi @pitrou, I’ve opened a PR to address #50735, which adds the missing run_end_encode and run_end_decode functions to the C++ compute documentation.

I’ve also added the corresponding documentation for run_end_encode. if you could take a look and let me know if any changes are needed. Thank you!