docs: fix the documentation link in package metadata

[j45856021-dev]

What does this PR do?

Point [project.urls].Documentation to https://lightning.ai/docs/litserve, matching the README's Docs link without its tracking parameters.

The current https://lightning-ai.github.io/litserve/ destination returns HTTP 404. PyPI's metadata for the published 0.2.19 release contains that same broken URL, so users following its Documentation link cannot reach the docs. This one-line correction fixes the link in future package releases; it does not change the metadata of an existing release.

No dependencies, runtime behavior, other project URLs or formatting are changed. This documentation-only correction uses the PR template's exception to prior issue discussion.

Validation

• A dependency-free TOML regression fails on the current base and passes after the change. Five controls pass before and after, covering the README destination, all other configuration/project links, published release metadata and the captured HTTP responses.
• Verified HTTP 404 for the old URL and HTTP 200 for the replacement. The replacement serves the documentation site's JavaScript shell; this is not a browser-rendering check.
• Verified that every source byte outside the replaced URL is unchanged. git diff --check passes.
• Draft pending package/schema CI: local python -m build --wheel --sdist and python -m pre_commit run --files pyproject.toml cannot start because those tools are unavailable. Dependency restoration also failed fetching Hatchling over verified HTTPS. No package build, repository test-suite or pre-commit success is claimed.

Before submitting

• Documentation-only improvement; prior issue discussion is not required by the template.
• Read the contributor guide and its linked Pull Request section.
• Updated the affected documentation link.
• Checked the changed metadata and unchanged-field controls.
• Package/schema validation and full pre-commit hooks.