docs: refresh README for Python 3.14 RC ecosystem updates

[koxudaxi]

This pull request updates the README.md to reflect the latest status and ecosystem support for Python's Template Strings (t-strings) as of Python 3.14 RC3. The changes clarify the feature's stability, improve resource links, update tool and library recommendations, and provide more accurate setup instructions for users wanting to try t-strings.

Feature status and documentation updates:

• Updated the warning to indicate that t-strings are now accepted and stable in Python 3.14 RCs, with general availability scheduled for October 7, 2025.
• Revised the feature list to clarify rendering control and the current state of type checking support.

Resource and toolchain updates:

• Replaced beta resource links with up-to-date documentation for Python 3.14 RC3, including the canonical string.templatelib docs, release schedule, and updated Docker/source references. [1] [2] [3]
• Updated type checker recommendations to point to upstream Pyright (no fork required) and note mypy's current behavior.

Library and ecosystem updates:

• Expanded the database and utilities sections with new and updated libraries that support t-strings, including t-sql, psycopg 3 preview, tdom, and tstring-util.

Clarification of related proposals:

• Updated the reference to PEP 787 to note that it is deferred to Python 3.15 and will propose t-string support for subprocess/shlex.