[EuiTour] Style and a11y fixes#5225

Merged

cchaos merged 6 commits intoelastic:masterfrom

cchaos:update/tour

Sep 30, 2021

Commits on Sep 29, 2021

[EuiTour] Made `subtitle` optional and fixed footer background
cchaos
committed
[Breaking] Removes boolean as an option for `minWidth`
cchaos
committed
Fix heading levels
cchaos
committed
More tests
cchaos
committed
cl
cchaos
committed

Commits on Sep 30, 2021

Better tests and switching to `h2` when no `subtitle`
cchaos
committed