Update clang-tidy rules; Tune severities

[ivangalkin]

Apply custom severities, suggested by @nathanaelg

ATTENTION! severity of the following exiting rule was lowered

• cppcoreguidelines-no-malloc

please review!

misc: original documentation contained internal hyperlinks, change them to external ones

#1536 ( still doesn't close the issue, we need more rules mentioned in #1560)

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This change is 

[guwirth]

@nathanaelg please add also your comments

[nathanaelg]

I had put cppcoreguidelines-no-malloc as info code smell because it was one of the "don't use this feature of the language" even though if you use it properly it isn't necessarily problematic.

On further thought though, it is one of those things that is fairly easy to get wrong and the consequences are fairly bad so might be best to have it as a major code smell - I'm still not convinced it should be classed as a bug rather than code smell.

Also, if you do change it, hicpp-no-malloc should match as it is an alias.