Add CARTA startup version validation

[izkgao]

Description

Closes #166.

Companion frontend PRs: CARTAvis/carta-frontend#2921

Prepare the carta-python 2.0 development cycle. This branch adds data-driven CARTA compatibility validation, validates CARTA 6.1+ during session startup, and documents the updated PyPI and development workflow.

What is implemented:

• Add startup CARTA version validation with configurable warning/error handling, compatibility suggestions, improved action errors, and backend cleanup when session creation fails.
• Start 2.0.0-dev, rename the distribution to carta-python, and add the data-driven compatibility table/version helpers.
• Update installation, quickstart, and development documentation, with tests covering session validation, compatibility behavior, error paths, and deprecation handling.
• Add PR template

Checklist

For linked issues (if any):

• assignee and labels added
• GitHub Project estimate added

For the pull request:

• reviewers and assignee added
• GitHub Project estimate added
• tests added or updated / no tests needed
• documentation updated / no documentation update needed

For dependencies and compatibility:

• companion frontend or backend PRs linked / no companion PRs needed
• pyproject.toml and uv.lock updated / no dependency changes
• CARTA compatibility table updated when the minimum CARTA series changes
  or dev starts a new carta-python major/minor series / no compatibility
  change

For pull requests targeting main:

• VERSION.txt finalized for the release
• final compatibility entry matches the release major/minor version and
  minimum supported CARTA series

[izkgao]

Previously, I thought we should limit support to a single CARTA major version, since deprecated frontend APIs could be removed in the next major release. However, now that we have decided carta-python should support multiple frontend API versions, this restriction no longer seems necessary.

Because CARTA typically has only a few minor releases within each major version, it makes more sense for a carta-python release to remain compatible with future CARTA versions until an actual breaking change occurs. I also plan to add frontend CI checks to detect such breaking changes.

I have updated the code and documentation accordingly.