Deprecrate legacy and seperate example galleries

[samaloney]

PR Description

• Deprecate legacy, add warning on module import and add warnings call outs in the documentation.
• Split gallery into Example and Legacy sections

[Copilot]

Pull request overview

This PR deprecates the sunkit_spex.legacy module and reorganizes the documentation by splitting the example gallery into separate "Examples" and "Legacy Examples" sections.

Changes:

• Added deprecation warning to sunkit_spex.legacy module that triggers on import
• Split example gallery into main examples and legacy examples directories
• Updated file paths in legacy examples to account for new directory structure
• Updated documentation configuration and intersphinx mappings

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
sunkit_spex/legacy/init.py	Adds deprecation warning using SunpyDeprecationWarning when the legacy module is imported
pytest.ini	Adds filter to ignore the legacy module deprecation warning during tests
pyproject.toml	Replaces pypandoc dependency with joblib for sphinx-gallery parallel execution
examples/legacy/*.py	Multiple legacy example files with updated file paths from ./ to ../../ to reflect new directory structure
examples/legacy/README.txt	Adds README with deprecation warning for legacy examples section
docs/reference/legacy.rst	Updates deprecation warning message to be more specific about version and timeline
docs/how_to/index.rst	Adds legacy gallery section to documentation table of contents
docs/conf.py	Modernizes import statement, updates intersphinx URLs, and enables parallel execution for sphinx-gallery
changelog/250.doc.rst	Documents gallery split
changelog/250.deprecation.rst	Documents legacy module deprecation

Comments suppressed due to low confidence (1)

examples/legacy/fitting_RHESSI_spectra.py:363

• The save filename path is inconsistent with the data file paths. Data files use ../../rhessi/ to go two levels up, but the save filename uses ../ which only goes one level up. This should be ../../sunxspexRhessiSpectralFitting.pickle to be consistent with the directory structure and match the pattern used in other examples like fitting_STIX_spectra.py (line 205).

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[KriSun95]

Other than the comment about multiple links it LGTM!

Happy to approve if the weird link issue is only on my machine too.

[samaloney]

Ok it's sorted now https://sunkit-spex--250.org.readthedocs.build/en/250/how_to/index.html

[KriSun95]

I didn't realise we could access the docs build from the tests in the PR 🤯

The sidebar menu now only shows the link to the Example Gallery. I got that behaviour when I just removed the link to ../generated/gallery/legacy/index in the docs/how_to/index.rst file instead of doing the :hidden: thing I suggested.

I suppose my question would be: do we want the behaviour that the legacy gallery is slightly more hidden? I could be convinced either way to be fair.

[samaloney]

Yea it's because the legacy gallery is a subgallery of the example gallery. I didn't want to have to create a 2nd subgallery as I'd have to name it Current or something. So I've used the approach @KriSun95 suggested for the moment.