Generate ToC with Hugo rather than JavaScript

[zecakeh]

Thanks to the .RenderShortcodes function added in Hugo 0.117.0, we can include the CS modules and all the headings are included in the generated ToC, with unique IDs. This avoids to do it with JavaScript in the browser.

This should fix the issue in #1841.

This might however leave some duplicate IDs in the code generated from OpenAPI definitions.

Pull Request Checklist

• [x] Pull request includes a changelog file
• [x] Pull request includes a sign off
• [x] Pull request is classified as 'other changes'

Preview: https://pr1851--matrix-spec-previews.netlify.app

[richvdh]

Writing it here for want of somewhere better: I wondered if we could avoid cloning the whole of sidebar-tree.html by putting toc.html back in baseof.html. The answer is that we can, with some custom CSS, along the lines of:

baseof.html:

-            {{ partial "sidebar.html" . }}
+            <div class="sidebar-div td-sidebar-nav">
+              {{ partial "sidebar.html" . }}
+              {{ partial "toc.html" . }}
+            </div>

Custom CSS:

.sidebar-div {
    overflow-y: auto;
    position: sticky;
    top: 4rem;
    height: calc(100vh - 5rem);
}

.td-sidebar__inner {
    height: auto !important;
    position: static !important;
}

(In other words, we have to move the height and position from .td-sidebar__inner to a higher-level div.) But frankly, that looks more brittle than just forking sidebar-tree.html.