Document the option terminator

[vivekjm]

Summary

• add a new guide for people using command-line tools built with ArgumentParser
• use the -- option terminator as the guide's first topic, with examples for dash-prefixed and option-like positional values
• link the guide from the documentation's Essentials section and keep the declaration article focused on defining command interfaces

Fixes #39

Testing

• git diff --check (passed)
• swift test --skip CountLinesExampleTests.testCountLines (passed; the skipped fixture is path-sensitive and fails because this checkout path contains spaces)
• swift package plugin generate-documentation --target ArgumentParser was attempted previously, but this toolchain reported that the plugin subcommand was unavailable

[natecook1000]

Thanks for looking at this, @vivekjm! This isn't the correct spot to document the -- behavior, since that article is about how to declare arguments in your command. We do need an article about using tools built with ArgumentParser. Would you like to take a crack at creating that, with this as the initial section?

[vivekjm]

Yes, thanks for the direction. I moved the detailed explanation into a new Using ArgumentParser Tools article, linked it from the documentation’s Essentials section, and restored the declaration article to its brief original note. The new guide starts with the -- behavior and includes both dash-prefixed and option-like positional-value examples.

I also reran the suite with the known path-sensitive CountLines fixture skipped: swift test --skip CountLinesExampleTests.testCountLines passes.