[feature] Access to filters and latest HTML from inside the `Notification` body and title

For example, you have set some change detection on the text output of a web-page but you also want to extract some special HTML from inside that latest HTML as part of your snapshot

But the page includes some special link which you want to include in the notification (maybe you even want to update some API, or just want it by email etc)

## Include the output of a filter by row

```
{{watch_url}} had a change - compare it with {{watch_url}}

Special link inside document was: {% extract("//a[contains(@href, 'lets-go.html')]/@href") %}

```

so if the document contains something like

```

Some text i'm watching

Click here

Then it would return the `@href` full link `https://example.com/lets-go.html?yes=12345`

Full example

Start by tracing how the Notification body and title are rendered and where filters are evaluated. Use the example extract expression and latest HTML requirement as the behavioral reference. Done means notification templates can access filter output and extract the requested link from the latest HTML.

Written by the indexing model from the issue text.