(contributing) Add guidelines for anchoring and aliasing of pip entries

[MatthijsBurgh]

As discussed in #28891

Fixes #28891

[tfoote]

Thanks for the followup. I think that this will work. The name is not completely self explanatory but having this document written out hopefully will help people find the full meaning.

One typo to fixup before merge.

[cottsay]

Could you add clarification to the document what date should be used in the anchor?

[nuclearsandwich]

In general I think we have to be very cautious when using advanced yaml features both because they're not implemented in all parsers (especially in safe modes) and because they aren't clear to casual yaml editors.

It's a bummer that we have to have the anchors before the aliases as it will create more churn when remove the python- key and move the content to the aliased python3- key.

I cannot think of a different way to solve this same problem and ultimately this is an issue of transition which means that we'll eventually be through it.

Well done @MatthijsBurgh on pushing through and finding a solution to a nasty problem.

[MatthijsBurgh]

@cottsay @ivanpauno friendly ping ;)

[MatthijsBurgh]

@cottsay @ivanpauno ping ;)

[nuclearsandwich]

@MatthijsBurgh thanks for iterating with our team to propose a solution. I'm glad we came up with something that while a bit rough, is no rougher than the problem space, and has an explicit expiration date.