Introduce a `sinceVersion` front matter attribute

Would be useful to be able to set something like
```yaml
sinceVersion: v7.2
```
in the front matter so a page can display when a feature was introduced. This could maybe be displayed next to the tags.

Start by locating the front matter parsing and the code that renders page tags. Trace how an existing metadata attribute reaches the page display, then verify that `sinceVersion` accepts values such as `v7.2` and appears next to the tags when present.

Written by the indexing model from the issue text.