docs(python): Draft New Spans and Migration Guide pages

[inventarSarah]

DESCRIBE YOUR PR

This PR adds the initial docs for Python span streaming: a New Spans page (intro to stream mode) and a Migration Guide (how to update existing custom instrumentation).

Context
For Python, the new Span API and stream mode ship together, so migrating to stream mode means migrating to the new Span API at the same time abd there's no way to use the new API while staying in transaction mode.
Because of that, putting the migration steps into the New Spans was way too much content and I split that content out into a new Migration guide. This guide is basically a human-readable version of the migration content from the engineering skill file.

Considerations

• Content here may still change once I go through the existing Tracing, Option, Filtering, etc. pages
• The Migration Guide is currently nested under New Spans in the nav. But it could also live under the main "Migration Guides" section instead. Let me know if it makes sense to move the guide.
• I reused content from the JS New Spans page -> I will create includes for these later when the content is fixed :)

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LEGAL BOILERPLATE

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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