Update tutorial to use httpx2

[dawnwang3692]

Updates Tutorial 8 to use httpx2 instead of httpx, including the dependency and client examples. Adds the Android certificate verification note requested in #89, updates the related custom-icons troubleshooting note, and adds truststore to the spelling wordlist.

Testing:

• tox -e docs-all
• git diff --check upstream/main...HEAD

Fixes #89

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool
  Assisted-by: OpenAI Codex

[freakboy3742]

I've tweaked some of the language in the admonition, but otherwise, this is great! Thanks for the fix!

[dawnwang3692]

Thanks for the review and for applying the follow-up fixes!