Add sphinx.ext.linkcode to link API docs directly to GitHub source

[viknesh-ai]

Replaces sphinx.ext.viewcode (which copies source into the built docs) with sphinx.ext.linkcode, so each "[source]" link on readthedocs points straight to the corresponding line range on GitHub, following the same pattern used by numpy and pandas. Falls back to the main branch when stumpy.version isn't a resolvable release version (e.g. editable installs).

Fixes #1081

Pull Request Checklist

Below is a simple checklist but please do not hesitate to ask for assistance!

• Read our Contributing Guide
• Referenced a Github issue (or create one if one doesn't already exist)
• Read and reviewed all of the comments in the Github issue that you've referenced (along with cross-referenced issues/pull requests) to ensure that the issue still requires a pull request
• Checked that the issue has not already been assigned to anybody else or is already being addressed in another pull request
• Left a meaningful comment on the original Github issue to discuss the detailed approach for your contribution and received confirmation from the maintainers before proceeding with this pull request
• Forked, cloned, and checked out the newest version of the code
• Created a new branch
• Made necessary code changes
• Installed black (i.e., python -m pip install black or conda install -c conda-forge black)
• Installed flake8 (i.e., python -m pip install flake8 or conda install -c conda-forge flake8)
• Installed pytest-cov (i.e., python -m pip install pytest-cov or conda install -c conda-forge pytest-cov)
• Ran black --exclude=".*\.ipynb" --extend-exclude=".venv" --diff ./ in the root stumpy directory
• Ran flake8 --extend-exclude=.venv ./ in the root stumpy directory
• Ran ./setup.sh dev && ./test.sh in the root stumpy directory and ensured that all tests are passing locally
• Check this box if AI code assistance was used to generate 15%+ of the code in this pull request

[gitnotebooks]

Review these changes at https://app.gitnotebooks.com/stumpy-dev/stumpy/pull/1177