Create support article for new mixed-content warning when downloading file

[stephendonner]

Description

Create support article for new mixed-content warning when downloading file.

(This new piece of UI was introduced when we picked up Chromium 92.)

Steps to Reproduce

1. new profile
2. launch Brave
3. load leaseweb.com/platform/network
4. scroll down and pick any file to download
5. right-click and choose Save File As... and click Save/OK
6. look at the bottom info bar
7. click on the ^ and choose Learn more
8. look at where the link goes

Actual result:

example	example

Expected result:

Something like Google's help content: https://support.google.com/chrome/answer/6261569?p=mixed_content_downloads&visit_id=637618169922962689-3127803141&rd=1

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.28.76 Chromium: 92.0.4515.93 (Official Build) nightly (x86_64)
Revision	6eb43ff7850a1d710c3f827a0555737c74edab5c-refs/branch-heads/4515@{#1378}
OS	macOS Version 11.4 (Build 20F71)

/cc @Brave-Matt (sorry, didn't know whom else to tag - punt if there's a better person/place for this!)