[documentation][config] Provide more mirrors than keil urls for cmsis packs if exists.

[entd]

Description

Documentation and default config only provides the default config pointing to keil.com server. As of now happens keil.com servers are down and working with it is pointless (not your fault thought).

What we can do better to avoid issues, to have alternative urls for keil.com in cmsis pack config. Or at least document possible aditional urls. If they exists of course. Single fail would not bring down entire system

Steps to Reproduce

1. Try searching info in documentation for aditional mirror of cmsis. At least I found none. If they exist at all?
2. There is no hints in default config about mirrors.

Expected behaviour: Config if main url fail should either automatically select next one or if automatic behaviour undesirable, atleast have aditional entry in cmsis pack urls for user to change manually. Also aditional mirrors if exists any should be documented

Actual behaviour: As stands now I failed to find any info even if cmsis alternative mirrors exist. Keil.com is down and entire build cmsys build pack system is useless because I cannot get pack for my board. Atleast it should be noted in documentation if alternatives exist or does not.

[ilg-ul]

if you know of existing mirrors, you can configure as many as you need in Preferences -> C/C++ -> Packages -> Repositories.