fix(docs): replace outdated whalesay image with busybox

[agilgur5]

Fixes #11858, Fixes #13388, Fixes #12767 (comment)

Motivation

The whalesay image was last updated 9 years ago and does not support Docker Manifest v2 nor arm64 architectures.
Per docker/whalesay#6, where I tweeted at Docker and emailed support twice, Docker has no intention to update the image (it just requires a rebuild, that's it... 🤷 ) nor even mark it as deprecated or archived... 😕

Modifications

• replace whalesay image with busybox image
  • also instead of cowsay, use plain echo
  • allow busybox in tested examples

secondary modifications

• rename several templates that were named whalesay unnecessarily as well

  • they often just printed a message, and so are now named print-message
  • others always printed "hello world", and so are now named hello-world
  • some didn't even use the whalesay image, and so are now named more accordingly

• also clarify a few other template names that would do more than print a message

  • now named print-message-from-file or hello-world-to-file

• plus make some small style adjustments per the docs style guide while at it

  • and remove some trailing newlines in codeblocks

Verification

• No more results for whalesay or cowsay in docs and examples
  • well, there are a few leftover, from the upgrading guide and the configmap, where whalesay is very intentionally used as a manifest v1 example. also a few cowsay ones leftover are all from the argosay image, see "Future Work" below
• Some of these, like the "hello world" doc, I manually tested
• The examples have an E2E suite that tests them

Future Work

1. Replace argosay as well and remove the argosay build entirely from the codebase
   • this does need a bit more testing just to make sure it's no longer needed with Emissary per fix: upgrade argosay image for security and build support arm64 #10435 #11613 (comment)
2. Replace both argosay and whalesay in all tests
   • there seems to be 1000+ references in the tests 😕
   • they don't seem to cause problems yet and aren't user-facing

[juliev0]

Thanks for doing this.

The examples have an E2E suite that tests all of them

Oh, nice. I was going to ask about that.

[agilgur5]

• The examples have an E2E suite that tests them

It actually didn't run here apparently since no source or test code changed 😕
I did run it myself though (hence the last commit), and it ran on later PRs.

Changing the diff detection is actually a little easier said than done though, as it wraps the whole E2E matrix, and this is only one of the test suites. The more correct way to do it would probably be to split off the examples from the matrix as its own job that runs if the e2es should run or if the examples changed. But that may require a pretty decent-sized refactor.

• Some of these, like the "hello world" doc, I manually tested

I've also been meaning to replace many of the docs examples with embeds to the examples/ dir. That would reduce duplication, help with single source-of-truth, aid discovery, and make them easily testable (if they are not existing duplicates), etc.