doc: flexibilize sphinx-rtd-theme allowed versions

Allow versions starting from 0.5.2 up to < 1.0, be the next major
releases. Major releases tend to include breaking changes, so better
restrict such releases until they are manually tested for potential
regressions.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
1 file changed