feat: Support httpx2 - #6463

We've added a new integration to the Python SDK in
getsentry/sentry-python#6463. This adds the docs
page for it. Note: It's just a copy of the existing HTTPX page as the
new package (HTTPX2) is a fork.

Preview:
- integrations table:
https://sentry-docs-git-ivana-add-httpx2-integration.sentry.dev/platforms/python/integrations/#http-clients
- integrations page:
https://sentry-docs-git-ivana-add-httpx2-integration.sentry.dev/platforms/python/integrations/httpx2/

## IS YOUR CHANGE URGENT?  

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## SLA

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## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [x] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

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## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

### Description

Support [httpx2](https://github.com/pydantic/httpx2).

Copy our httpx integration and tests and use that as the basis for the
new integration, so that if the new fork diverges, we can only edit the
dedicated integration.

Changes compared to `HttpxIntegration`:
- Renamed all httpx -> httpx2 (duh)
- Needed a replacement for `pytest-httpx`. Picked `httpx2-pytest` since
it's drop-in. Since it's a new project, pinned it to the current version
for additional safety.
- The tests needed adapting to be 3.14 compatible (httpx isn't tested on
3.14): `asyncio.get_event_loop().run_until_complete` -> `asyncio.run`

Docs: getsentry/sentry-docs#17940

#### Issues
- Closes getsentry#6448
- Closes
https://linear.app/getsentry/issue/PY-2502/add-support-for-httpx2

#### Reminders
- Please add tests to validate your changes, and lint your code using
`uv run ruff`.
- Add GH Issue ID _&_ Linear ID (if applicable)
- PR title should use [conventional
commit](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors:
[CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)