feat(python): Reintroduce Python span streaming docs

[sentrivana]

This reverts commit ba13b79.

We initially merged the streaming docs too early, so we had to revert. This PR reverts the revert and reintroduces the streaming docs again.

Changes on top:

• fixing sampling context examples as the format has changed (to be consistent with the old format)
• getting rid of all _experiments mentions as the options are now all top level (except for before_send_span, but we'll make a release with that before we merge this)

DO NOT MERGE. We'll merge once ready.

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[github-actions]

⚠️ Redirect Chain Issues Detected

This PR introduces or contains redirect chains that degrade SEO and add latency.

Content Links Pointing to Redirects (1 found)

These links point to URLs that redirect. Update them to point directly to the final destination:

File	Line	Current Link	Should Be
docs/platforms/python/tracing/configure-sampling/index.mdx	607	/platforms/python/configuration/sampling/#sampling-context	/platforms/python/sampling/#sampling-context

---

Each redirect hop loses ~15% of SEO link equity and adds latency for users.

[coolguyzone]

Looks good!

[sentry]

Bug: The code directly accesses span["attributes"], but the attributes key is optional. This will cause a KeyError if a span without attributes is processed.
_{Severity: CRITICAL}

Suggested Fix

Before accessing span["attributes"], add a defensive check to ensure the attributes key exists. If it doesn't, you can either return early or initialize it as an empty dictionary, for example: if "attributes" not in span: span["attributes"] = {}.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: platform-includes/configuration/before-send-span/python.mdx#L10-L11

Potential issue: The code examples for the `before_send_span` callback access
`span["attributes"]` without first checking if the `attributes` key exists. According to
the SDK specification, the `attributes` key is optional. If a span without attributes is
processed by a user's callback implementing this example code, it will raise a
`KeyError: 'attributes'`, causing a runtime error in the user's application. This can
prevent spans from being processed correctly.

Also affects:

• docs/platforms/python/tracing/streamed-spans/index.mdx:328~329