[CIVIS-5427] DOC pin mermaid version for graph rendering in Sphinx docs

[jacksonlee-civis]

The upstream mermaid has had a change that breaks sphinxcontrib-mermaid we're using to render the graph/network in the Sphinx docs. See mgaitan/sphinxcontrib-mermaid#110

This PR fixes the problem by pinning the mermaid version.

Currently / before this PR:

After this PR (screenshot from my local Sphinx HTML build):

This PR does not require a new PyPI or GitHub release.

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[gbolly]

LGTM