boostrap-new-os.md doc is outdated or obsolete

[mthalman]

The doc at https://github.com/dotnet/source-build/blob/main/Documentation/boostrap-new-os.md looks to be outdated or perhaps entirely obsolete. For example, it refers to a buildbootstrapcli.sh script that doesn't exist anymore. It should be examined to determine whether any of the content is relevant anymore and updated as appropriate.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[blucybrb14de]

(See above issue, for details.)

As mentioned in #4725, it would be great if I could get some clarification, for building the bootstrap. I am trying to fix dotnet-sdk, support for Gentoo GNU/Linux x86_64, using LibreSSL, as non-offical support for those systems, and it would be really great, if I could provide that that as a fix.