docs: updates to landing page and getting started

[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• The CodePen blank demo is out of date and we can no longer update it after stopping payment for the Pro plan.
• The "bucket" links on the home page have excess margins that break their layout flow.
• We link to some out of date versions of material-start.
• We don't link to our StackOverflow tag.
• CodePen embeds on the layout troubleshooting page have extra space at the bottom of the white frame.

Issue Number:
N/A

What is the new behavior?

• fix ul/li styles for "buckets" used for lists of links or videos
• add Blank StackBlitz Demo starter
• update issue template to use new StackBlitz blank demo and reduce passive voice
• remove out of date CodePen starter that can't be updated anymore
  • since we stopped paying for the Pro license
• remove links to unmaintained and out of date versions of material-start (es5, es6 + JSPM)
• add AngularJS Material ngUpgrade example with the Angular CLI
• remove unnecessary section with redundant links to Material Design
• add a couple more conference video links
• add link to StackOverflow angularjs-material tag
• update list of supported browsers
• fix CodePen embeds on layout troubleshooting page
  • having extra space at the white frame bottom

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[jelbourn]

LGTM