Document the header layout migration and update README examples

[ac1982]

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes. (Documentation-only; compiled and ran the three README examples.)
• API of new functions and classes are documented. (No new API.)

Description

Fixes #2851.

Users upgrading from 0.25 encounter missing headers such as xtensor/xarray.hpp, including in the README examples. Add an installation-guide section with common old/new paths, guidance for locating other headers and keeping the correct include search root, and the C++/xtl requirements across the transition. Update the README examples and the documentation index's obsolete C++14 requirement.

Validated all three README examples with GCC 15 and Clang 21 against 0.26 (C++17) and current master (C++20), using xtl 0.8.0. Original includes fail in all twelve combinations; updated examples compile and produce the expected numerical results. Checked all sixteen mappings against 0.25, 0.26 and current headers. Changed-file pre-commit checks pass.

Built the HTML documentation with Doxygen 1.15 and Sphinx: the final build and unmodified baseline produce the same fifteen warnings, so neither passes warnings-as-errors. Verified both new link anchors in generated HTML. Full C++ unit tests and other platforms were not run for this documentation change.

Prepared with OpenAI Codex assistance.

[JohanMabille]

Thanks!

[JohanMabille]

Documentation only, no need to run the CI.