Add docs for GetTopologyPath

[mattlord]

Companion PR to vitessio/vitess#15933

Review link: https://deploy-preview-1757--vitess.netlify.app/docs/20.0/reference/programs/vtctldclient/vtctldclient_gettopologypath/

This also brings all of the vtctldclient docs up to date with:

export COBRADOC_VERSION_PAIRS="get_topo_path_version:20.0"
export VITESS_DIR=~/git/vitess
make vtctldclient-docs

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