Replace/remove codeplex links

[mairaw]

CodePlex is being completely shut down in December. For more info, see https://aka.ms/codeplex-announcement.

We need to replace existing text and links that talk about codeplex:
https://github.com/dotnet/docs/search?utf8=%E2%9C%93&q=codeplex&type=

[doddag]

Hi Mairaw

I would like to contribute to this issue, and this would be 1st time i'm trying to contribute.

Do you have any advise for me?
Links / Text referring to CodePlex

[mairaw]

Hi @doddag, this is great! I see that you already submitted a PR. I'll go through it now and leave you feedback. Thank you so much!

[isaacrlevin]

@mairaw @doddag is this closed?

[mairaw]

No, @isaac2004. @doddag took care of a few hits only so this is still up-for-grabs.