Improve configuration to create front page

[chrisjsewell]

Heya, so in https://aiida-website.readthedocs.io/ I've played with pydata-sphinx-theme v0.12.0rc1 to create a nice splash page, that keeps the standard layout (and top bar), but is properly full-width and, combined with sphinx-design, to make it size responsive. (you'll note it looks very similar to https://code.visualstudio.com/, which is certainly not a bad thing!) I think maybe some of the things I'm doing could be up-streamed, to make it easier for others (this is essentially a follow on from #146)

The main steps were:

1. Remove any sidebars: this is already possible with html_sidebars and html_theme.sidebar_secondary.remove config, but maybe could be streamlined

2. Apply page specific CSS: I did this using a local extension and a html-page-context event, but could be improved

3. Remove max-width restrictions on page: I applied in my page specific CSS, it would certainly be ideal to upstream this in some way:

    .bd-container__inner.bd-page-width {
        max-width: 100% !important;
    }
   
    .bd-article-container {
        max-width: 100% !important;
        padding: 0 !important;
    }
   
    .bd-article {
        padding: 0 !important;
    }
   

4. Hide the page title

5. Now add sphinx-design elements