Docs/CSS: Fix Notes/Warnings blocks' BG colour to improve its state.

Docs' BG was original "#fff" and blocks such as code blocks or
warning-notes blocks were coloured in #F2F2F2.

In order to make it easier to read everything, the BG colour was
changed to #fff. However, the switch missed to convert other blocks'
BG colour.

This commit aims to re-introduce the correct contrast by changing the
BG colour of warning/notes block to #fff.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5441 from ostezer/docs-fix-notes-warnings-bg

Docs/CSS: Fix Notes/Warnings blocks' BG colour to improve its state.