[dart 3.10.0] Add dart-install guide

[antfitch]

Adds a guide for dart install to Tools & techniques section.

Fixes 6688

Previews:

• dart install guide: https://dart-dev--pr6725-dart-install-guide-tua1w3yq.web.app/tools/dart-install
• update to the pub guide: https://dart-dev--pr6725-dart-install-guide-tua1w3yq.web.app/tools/pub/cmd/pub-global#activating-a-package

Notes:

@dcharkes, please review this first and add your changes directly to the files if they are small.
@parlough, as soon as Daco is finished, I'll send this your way for a final review.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• This PR doesn't contain automatically generated corrections or text (Grammarly, LLMs, and similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn't use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 3580641):

https://dart-dev--pr6725-dart-install-guide-tua1w3yq.web.app

[dcharkes]

Thanks @antfitch! 🙏

We can't merge this yet, I'm still working on a dart install implementation. Once I've implemented it, it will only be available in dev releases (and then the next beta).

I've made the edits that I'm aware of.

[dcharkes]

FYI: dart install is now available in the dev releases.