docs(calendar): add calendar demo

[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?

[ ] 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?

The md-calendar component has no demo.

Issue Number:
N/A

What is the new behavior?

The md-calendar component has a demo.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

this is needed as there are some bugs in this area
it needs to be easier to submit reproductions