chore(docs): cope with trailing slash and "index" in URLs

Closes #6295
docs($interpolate): correct small typo
docs(guide/compiler): remove innacurate statement

The $compile service accepts string as a value and wraps it if
needed, so this statement isn't correct, at least by the source
code.

Closes #6301
test(docs): add check for resilience to trailing slashes, etc.