Documentation omnibus

The recent move to Antora for Unordered docs has left some minor glitches that we compile in this omnibus issue:

**1. Reference sections have "Untitled ..." tab name.** See for instance [here](https://www.boost.org/doc/libs/develop/libs/unordered/doc/html/unordered/reference/unordered_flat_map.html):


This is due to the fact that reference pages have no main header (note the first subsection begins with `==`), which, in the context of Antora, results in the page having no title:

```
[#unordered_flat_map]
== Class Template unordered_flat_map

:idprefix: unordered_flat_map_

`boost::unordered_flat_map` — An open-addressing unordered associative container[...]
```

There are two possible approaches to fix this:

* Somehow assign a title to the page that serves to compose the tab name while still not displaying on the page proper. I've experimented with attributes `:notitle:`, `:showtitle!:` etc., to no avail. See [this conversation](https://antora.zulipchat.com/#narrow/channel/282400-users/topic/Hiding.20title.20of.20a.20page) on Antora's support forum.
* Move all level _n_ sections to level _n_-1. This looks ugly, as the headers appear too big for my taste.

**2. Mobile devices: font size too big in some parts of the reference.** See for instance [this](https://www.boost.org/doc/libs/develop/libs/unordered/doc/html/unordered/reference/unordered_flat_map.html#unordered_flat_map_try_emplace) as it appears on my iPhone 8:

This seems to happen when inserting a block of code inside a "Notes:" construct:
https://github.com/boostorg/unordered/blob/3d2c63f744809cdb4d41047329fc9c4893b6cc8d/doc/modules/ROOT/pages/reference/unordered_flat_map.adoc?plain=1#L913-L940

**3. Mobile devices: two clicks needed to go to a TOC section.** Tested on my iPhone 8: go to the TOC and click on any entry (in the images below, "Hash Quality") --> the entry becomes underlined and a further click is needed to go through it: