Add Span links specification and envelope payload changes

[Lms24]

Which section?

SDKs -> everything related to span, trace context and the respective envelope payloads

Page URL

No response

Description

In RFC #141, we agreed on adding support for span links to transactions and child spans. Let's make sure to reflect this in the develop docs as well so that they're up to date and complete(ish).

Suggested Solution

Places to update:

• https://develop.sentry.dev/sdk/data-model/event-payloads/contexts/#trace-context
• https://develop.sentry.dev/sdk/telemetry/traces/#new-span-and-transaction-classes
• https://develop.sentry.dev/sdk/data-model/event-payloads/transaction/

Optionally: Create a page for span links in sdk/telemetry/traces/[Span Links]