Reduce the number of formats built on readthedocs to avoid timing out

[blnicho]

Fixes N/A

Summary/Motivation:

Following some recent online doc changes (including more extensive library references and improved enum documentation) we are now hitting the build time limit on readthedocs. This PR reduces the number of downloadable formats that we build to get us back under the time limit.

Changes proposed in this PR:

• Only build the pdf downloadable format on readthedocs

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