Propagate PIP_INDEX_URL, PIP_EXTRA_INDEX_URL
to the dependency-scanning Docker image
to support Python projects depending on custom
Pypi registries. These variables will be consumed
by the gemnasium-python analyzer.

Removing extra blank lines in docs that serve
no purpose, as well as some --- lines that also
are not needed.

The manual job definitions are deprecated and are not
guaranteed to work as expected. We should not confuse
our users with including them in the docs.

This deletes all the deprecated sections and cleans up
the configuration section significantly.

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

Addresses the following questions:

- How often do we update the vulnerability DB or analyzers?
- Do we have to update GitLab to benefit from latest vulnerabilities
  definitions?

Port all info from:

- security-products/dependency-scanning/blob/master/docs/README.md
- security-products/dependency-scanning/blob/master/docs/analyzers.md