GH-50744: [R] Add read_ipc_file and write_ipc_file to _pkgdown.yml reference index #8578

Job	Run time
check-labels / Check labels	2s
check-enabled	2s
matrix.title	-1s
ARM64 Windows 11 MSVC	-1s
ODBC Windows	-1s
ODBC ${{ matrix.build-type }} ${{ matrix.architecture }} macOS ${{ matrix.macos-version }}	-1s
JNI Windows	-1s
JNI macOS	0s
ODBC Linux	-1s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	0s
ODBC nightly	0s
report-extra-cpp	0s
	-2s

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