Create a new page with info about OS portability

[AlexDaniel]

Part of https://github.com/perl6/doc/issues/657:

Tutorials:
    * OS portability details

But what kind of details?

[AlexDaniel]

For example, details on how \r\n and \n are handled.

[JJ]

In a new page?

[AlexDaniel]

Preferably, yes. I think it would be better to have all OS-related info on one page.

[raiph]

"It may also be worth mentioning in some Raku document somewhere that distributions should not contain symlinks if they are expected to be platform agnostic.".

Note that this was prompted by an installation issue that arose for a Raku distribution that was shared, not an issue with running Rakudo or a Raku program once a package had been installed, but I agree with @AlexDaniel that it would be better to have all OS portability info on a single page, at least until it's huge. (And given that it's remained non-existent for 7 years, I'm thinking we don't need to worry about that for now!)