Add section header link copy to docsite

[nynexman4464]

I want to link to the section "Add the theme CSS" but there 1) isn't any button to do so and 2) no anchor tag or ID to actually allow us to do that

https://astryx.atmeta.com/docs/getting-started

[ernestt]

Addressed in #3101.

Doc section headings now have stable id anchors plus a hover "copy link" button, so you can link directly to a section (e.g. #add-the-theme-css on Getting Started). The package/README pages get heading ids too.