Documentation for nonlinear least squares (NLS) problems

[arnavk23]

Closes #162
@dpo @tmigot

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.34%. Comparing base (2ada450) to head (eaa79f9).
⚠️ Report is 46 commits behind head on main.

Additional details and impacted files

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##             main     #411       +/-   ##
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- Coverage   99.80%   54.34%   -45.47%     
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  Files        1084     1138       +54     
  Lines       12319    13371     +1052     
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- Hits        12295     7266     -5029     
- Misses         24     6105     +6081     

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[Copilot]

Pull request overview

This PR updates the documentation to better explain how nonlinear least squares (NLS) problems are represented and accessed in OptimizationProblems.jl, and fixes a couple of tutorial typos.

Changes:

• Add NLS user-facing documentation to the tutorial and metadata pages (including how to filter NLS problems and query *_nls_nequ).
• Add contributor guidelines for implementing NLS problems.
• Fix minor wording/typos in existing docs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
docs/src/tutorial.md	Adds a new NLS section describing use_nls=true usage and discovery via meta.
docs/src/meta.md	Documents the :objtype == :least_squares classification and get_*_nls_nequ() getters; adds filtering example.
docs/src/contributing.md	Adds contributor guidance for adding NLS problems (metadata + getters + construction pattern).

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