Review z-index for Modal

[mbohal]

When migrating to MkDocs/Docoff based docs (#440) an issue appeared with z-index preference. The MkDocs sticky header covers Modal which is wrong.

Expected behavior: Modal covers the sticky header, not vice versa.

How to reproduce:

1. Go to http://localhost:8000/components/Modal/#position
2. Modify the code by adding the attribute preventScrollUnderneath="off" to the Modal component
3. Click the Launch modal at top button
4. Scroll to the top of the page

[hubacekj]

It is caused by different stacking contexts for the sticky header and for the Modal. Both have their own stacking contexts caused by setting position: sticky and transform css property respectively. There are two immediate (one future) solutions:

1. Stop using transform property and replace it with negative margins (requires a javascript resize listener - therefore I wouldn't prefer it)
2. Use portal above all other elements in the documentation
   (3.) Use a native dialog element

[adamkudrna]

Point 3 relates to #461.

[adamkudrna]

Just adding a couple of screenshots: