chore: improve mktree method and update docs to help transition to RNTuple

[ariostas]

This PR improves the mktree method so that it accepts data instead of having to first us it with type data and then use extend to write the data. This makes it a much simpler replacement for the old syntax. I also updated the documentation and tests.

Closes #1504

[Copilot]

Pull Request Overview

This PR improves the mktree method to accept data directly, eliminating the need for a two-step process (creating with types then extending with data). This simplifies the migration from the deprecated file["tree"] = data syntax to file.mktree("tree", data) as Uproot prepares to default to writing RNTuples.

Key changes:

• Enhanced mktree to accept either type specifications or actual data
• Updated documentation to reflect the new recommended syntax
• Migrated all test files from deprecated assignment syntax to mktree

Reviewed Changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/uproot/writing/writable.py	Added logic to detect data vs types in mktree, extracted helper functions for type checking and data unpacking
src/uproot/writing/identify.py	Refactored to use new helper functions from writable.py, updated deprecation warning message
pyproject.toml	Removed filter for RNTuple deprecation warning (tests now use non-deprecated syntax)
docs-sphinx/basic.rst	Updated documentation examples to use mktree instead of assignment syntax
tests/*.py	Converted all test cases from file["tree"] = data to file.mktree("tree", data)
tests-wasm/*.py	Added comments explaining pytest_pyodide compatibility requirement

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[ianna]

@ariostas - Great! Thanks.