docs: fix View Demo button for directives w/o a demo. update icons, Bower references

[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?

• We display a Bower icon on the landing page along with an "Install with Bower" tooltip, but Bower has been deprecated for years.
• We use an old, out of date icon for GitHub and NPM on our landing page.
• The 'View Demo' button appears for the md-autofocus directive which doesn't have a demo.

Issue Number:
N/A

What is the new behavior?

• remove Bower references
• update GitHub and NPM icons
• update README.md
  • fix typos
  • update example versions
  • remove references to Bower
• fix 'View Demo' button appearing for directives w/o demos

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Before

After