Disable parallel documentation build

[adriangonz]

What this PR does / why we need it:

It looks like the sphinx_search.extension plugin that was added recently to the docs (#3433) doesn't support building the docs in parallel.
The build itself works, but it will trigger a warning that makes our CI/CI pipeline fail.

To account for the above, this PR disables the parallel build (by removing the -j auto flag) on the make html docs target, which is the one used in our CI/CD pipeline.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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