docs(dspy): fix the optimizers overview link on the BetterTogether page (#10253)

* docs(dspy): fix the optimizers overview link on the BetterTogether page

The "Further Reading" link pointed at ../../../learn/programming/optimizers.md,
which is wrong twice over: the optimizers overview lives under
learn/optimization/, not learn/programming/, and from docs/api/optimizers/ the
three-level prefix climbs above the docs root. mkdocs left the href untouched in
the rendered page, so the link 404s for readers.

Point it at ../../learn/optimization/optimizers.md, matching the two-level form
already used in api/utils/configure.md and api/utils/context.md.

* docs(dspy): point the further-reading link at the live optimizer guide

The first pass only fixed the path, so the link resolved but landed on a
redirect stub: mkdocs.yml maps learn/optimization/optimizers.md to
getting-started/gepa-optimization.md, and the whole learn/ tree redirects into
getting-started/. A reader clicking "DSPy Optimizers Overview" ended up in the
GEPA tutorial instead of an overview.

Link diving-deeper/choosing-an-optimizer.md instead, which is live in the nav,
carries no redirect, and covers what each optimizer tunes and how to pick one.
The label now repeats that page's own title so the two cannot drift apart again.

Author: saime428

docs/docs/api/optimizers/BetterTogether.md

@@ -149,4 +149,4 @@ Example optimizer combinations:
 
 - [BetterTogether Paper: arxiv:2407.10930](https://arxiv.org/abs/2407.10930)
 - [Databricks Case Study](https://www.databricks.com/blog/building-state-art-enterprise-agents-90x-cheaper-automated-prompt-optimization) - Real-world application combining BetterTogether with GEPA
-- [DSPy Optimizers Overview](../../../learn/programming/optimizers.md)
+- [Optimizers: choosing one](../../diving-deeper/choosing-an-optimizer.md)