[Website] Add blog post on Python UnboundedSource and Watch

[Eliaaazzz]

Adapt the GSoC final report into a concise blog post about the Python UnboundedSource API and Watch transform. Cover their design, runner validation, and local benchmarks, with a link to the full report.

Validation: checked against the report and current APIs; Hugo 0.117.0 production build, browser preview, and git diff --check passed.

[github-actions]

Assigning reviewers:

R: @damccorm for label website.

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[damccorm]

Thanks - adding @Abacn as a reviewer who is closer to the work here

[damccorm]

Are you comfortable including an email? In general, we try to have at least that information so that people can reach out if they have thoughts or questions (admittedly this is rare)

[damccorm]

In your intro, I think it would be nice to mention that this is a Google Summer of Code project - I think it both gives useful context and highlights your contribution a little bit better

[damccorm]

Also, it would be nice to address the "why?" here - what problem(s) were you trying to address