menu(dense): add demo and docs. fix min-height when scrollable

[Splaktar]

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[x] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

When using md-dense class with md-menu-content and there are enough options to cause scrolling, the items are overlapping and jammed together.

Issue Number:
Fixes #11278. Closes #11279.

What is the new behavior?

When using md-dense class with md-menu-content and there are enough options to cause scrolling, the items are properly spaced vertically with a min-height of 32px instead of 0.

There are new docs and a demo of using md-dense with md-menu.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information