docs: Migrate examples and guides from httpx to httpx2

[vdusek]

The SDK's examples and guides still reached for the original httpx, which has had no release
since 0.28.1 in December 2024. Its successor httpx2 is at 2.12.0 and is where the maintainers
went, so the docs were pointing readers at the dormant library.

Swaps the import in the Actor docstring and in the three doc code examples, then updates the
prose and links in the BeautifulSoup guide, the proxy management page, the uv guide, and the README.
The two renamed code examples and the renamed guide file keep their 01_/05_ ordering prefixes.

The guide keeps id: beautifulsoup-httpx, so /docs/guides/beautifulsoup-httpx still resolves.
@docusaurus/plugin-client-redirects is declared in website/package.json but registered nowhere,
neither in docusaurus.config.js nor in the shared docs theme, so renaming the id would dead-link
the old URL.

On the SDK's own dependencies: Crawlee and the Apify API client both default to impit, and
neither pulls in httpx or httpx2. Their HTTPX backends are optional extras that now resolve to
httpx2 (crawlee[httpx] wants httpx2 >= 2.12.0, apify-client[httpx2] wants httpx2 >= 2.0.0).
This change affects what the docs recommend to readers; the dependency tree is untouched.

One caveat worth knowing before merge: the guide says the BeautifulSoup Actor template ships both
libraries preinstalled, which holds once actor-templates migrates too. Until then the explicit
pip install beautifulsoup4 httpx2 in the same section covers anyone starting from the template.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.75%. Comparing base (e0801c9) to head (d6d7525).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1115   +/-   ##
=======================================
  Coverage   92.75%   92.75%           
=======================================
  Files          53       53           
  Lines        3519     3519           
=======================================
  Hits         3264     3264           
  Misses        255      255           

Flag	Coverage Δ
e2e	32.73% <ø> (ø)
integration	53.33% <ø> (ø)
unit	89.34% <ø> (ø)

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