53738105 manual html pub numbering

Editors can now mark their HTML versions as "manually numbered". Manually numbered HTML version headings can then be numbered as required using a numbering scheme that reflects how we currently list numbered headings, e.g:

```
## 1. Heading one

## 2. Heading two

### 2.1 Sub-heading

### Unnumbered sub-heading
```

Pivotal https://www.pivotaltracker.com/story/show/53738105