DOC: Migrate to `pydata-sphinx-theme`

[agriyakhetarpal]

Description

This PR prepares the transition of the PyWavelets documentation towards interactivity through in-line usage examples, which will be added in follow-up PRs through the integration of JupyterLite notebooks.

Several changes have been introduced, including but not limited to:

1. Replacement of the "nature" theme by the PyData Sphinx Theme, prevalent amongst Scientific Python packages
2. Two useful utility-based extensions, sphinx-togglebutton and sphinx-copybutton were added
3. The HTML theme options based on the available ones in the theme were added to customise various sections of the webpages (sidebars, navbar, footer, etc.)
4. Removal of older templates used to customise the sidebars and their icons
5. a few minor improvements to the directives present in the documentation

[agriyakhetarpal]

Since we do not have RTD checks for now, I am happy to add another entry to the PR workflows where we can run the doctests, build the doc, and check if they are warning-free – if this would be needed

[agriyakhetarpal]

For our interactive documentation goal, should I open an issue and add a task list there about the other deliverables, which shall be accommodated in further PRs?

[rgommers]

Since we do not have RTD checks for now, I am happy to add another entry to the PR workflows where we can run the doctests, build the doc, and check if they are warning-free – if this would be needed

That sounds nice, thank you. It would avoid merging things that then not build on RTD later.

[rgommers]

For our interactive documentation goal, should I open an issue and add a task list there about the other deliverables, which shall be accommodated in further PRs?

Sure, a single tracking issue would be helpful.

[agriyakhetarpal]

Thank you for your input – all tests are passing except for the AppVeyor build which is pending, this should be ready for a review now.

[rgommers]

The merge of gh-687 also caused a few merge conflicts - could you please fix those too @agriyakhetarpal?

[agriyakhetarpal]

The doc build shows one warning, could you check this?

For some reason I didn't see this one locally; we can keep navigation_with_keys as False, since that is what I assume the documentation for other libraries will be doing (or adapting to). I addressed it in https://github.com/PyWavelets/pywt/pull/704/commits/8ecb30b98fbb8afabbf22cdb79757e329f238359.

The pages all look good except for the "Signal extension modes" one which has a rendering issue:

This page in specific had an indentation issue. I addressed it in https://github.com/PyWavelets/pywt/pull/704/commits/1a383598151b6303567d305b3f7c80b92dfe6eea.

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P.S. I merged the master branch instead of rebasing because I didn't want to blemish your review. I am happy to rewrite the commit history or get this squash-merged – it is up to you :)

[agriyakhetarpal]

I fixed up all the tests (not sure if the doctests are actually supposed to print nothing or that is a bug?)