fix search through bumping the underlying RTD theme that fixed the issue

[imrehg]

The search issue was due to Sphinx changes and fixed in readthedocs/sphinx_rtd_theme#1021
that was released in the 0.5.1 version of the theme, so that's the earliest we can bump to to have functioning search again.

Looking at versions since then, the latest (1.0.0) version seems to have identical results in the compiled documentation, so
bumping to that version is likely more future proof.

[acroz]

I pinned this dependency as some of the styling changes in the RTD theme broke the CSS hacks in this repo. Have you run this with a more complex docs site e.g. the Faculty platform docs? I think if you do you may find that some of our style overrides are broken.

[imrehg]

@acroz I've tried the current docs (normal working pages) and the updated version side by side before doing this PR but after retesting it, I do see the sort of breakages you mean (especially in the documentation sections it's quite clear, headers spanning full length of page rather than being boxes)

Sorry about that being sloppy.

I have some ideas for doing a more proper search fix, let's close this one for now until that one is clearer.