Use ReadTheDocs

Currently we're using Github Pages for hosting and Netlify for PR previews. We keep having issues due to Netlify's Python environment, and it's lack of support for Conda making our packages fall out of sync.

ReadTheDocs now supports Conda/Mamba, so we could simplify our environment, and have the same service manage the build and PR previews.

I remember we had some discussion about RTD before, and I can't remember exactly why we didn't use them and instead built our own versioning scheme. Was it due to the advertising? We can opt it out of advertising by someone getting a membership, which I just tried for Xpublish.

To get our existing old sites, we would probably want to tag the desired commits, then use post_build customization to pull those in. The other place to customize might be to use single version mode.

xref: #237 @emiliom @ocefpaf

首先检查当前的 GitHub Pages 和 Netlify 配置，然后阅读 #237 中之前的讨论，以及链接的关于 Conda、post_build 和 single-version mode 的 ReadTheDocs 文档。完成标准是需要有一份经各方同意的迁移计划，涵盖构建、PR 预览和现有的旧站点；未指定实现文件或测试。