GH-50744: [R] Add read_ipc_file and write_ipc_file to _pkgdown.yml reference index

[thisisnic]

Rationale for this change

Errors due to functions missing from pkgdown docs

What changes are included in this PR?

Adds them

Are these changes tested?

Will run CI

Are there any user-facing changes?

No

• GitHub Issue: [R] Add read_ipc_file and write_ipc_file to _pkgdown.yml reference index #50744

[thisisnic]

@github-actions crossbow submit -g docs

[Copilot]

Pull request overview

Updates the R package’s pkgdown reference index so read_ipc_file() and write_ipc_file() appear in the generated documentation site, addressing missing-function doc errors.

Changes:

• Added read_ipc_file to the “Read files” reference section.
• Added write_ipc_file to the “Write files” reference section.

[github-actions]

Invalid group(s) {'docs'}. Must be one of {'ruby', 'fuzz', 'python', 'vcpkg', 'homebrew', 'wheel', 'nightly-packaging', 'verify-rc-source', 'packaging', 'example', 'nightly', 'verify-rc-source-macos', 'cpp', 'verify-rc', 'example-python', 'integration', 'c-glib', 'nightly-tests', 'verify-rc-source-linux', 'r', 'test', 'nightly-release', 'example-cpp'}
The Archery job run can be found at: https://github.com/apache/arrow/actions/runs/30559987190

[raulcd]

@github-actions crossbow submit preview-docs

[raulcd]

@github-actions crossbow submit test-debian-13-docs test-r-devdocs

[raulcd]

Probably those last two better than preview-docs :)

[github-actions]

Revision: b1e64ce

Submitted crossbow builds: ursacomputing/crossbow @ actions-77b235368e

Task	Status
test-debian-13-docs
test-r-devdocs

[github-actions]

Revision: b1e64ce

Submitted crossbow builds: ursacomputing/crossbow @ actions-8fff552a4b

Task	Status
preview-docs

[raulcd]

Thanks @thisisnic