docs: expand layout sizing controls

[Kkkakania]

Summary

Expand the Control Size how-to guide with the sizing controls that were previously used or available but not explained:

• describe min_width, max_width, min_height, and max_height and how they interact with responsive dimensions
• document project-wide defaults through pn.extension(sizing_mode=...) and pn.config.sizing_mode
• explain all width_policy / height_policy values and their precedence over sizing_mode
• add runnable examples for bounded responsive width and a maximum-width row

Closes #5381.

Validation

• Executed both new Panel examples locally and asserted their sizing parameters
• uvx pre-commit run --files doc/how_to/layout/size.md

---

AI disclosure: this documentation update was AI-assisted. I checked the behavior against Layoutable, executed the examples, and ran the repository pre-commit hooks.

[Kkkakania]

CI note: the failed UI jobs are unrelated existing failures in Tabulator header-filter/frozen-column behavior, Gauge updates, and the server index proxy test. This PR changes only the layout sizing documentation. Pre-commit, core, unit, and the remaining UI matrices pass; there is no layout-doc code change that would address these failures.

[philippjfr]

Great explanation, and test failures indeed unrelated. Merging, thanks @Kkkakania!