Move hidden search keywords to frontmatter when CHM+PDF go away

The search functionality is enhanced by the addition of some hidden keywords. Although they are hidden, and don't display when the page is viewed, they do show in the search results. They are present at the end of pages, so are not always visible in the search results as that is only an extract of the page on which the search term is found, but ideally they would never be displayed in the search result extracts.

Some initial investigation by xpqz (see below) indicated that this might be resolvable once the CHM output has been replaced; this issue will be investigated then – hopefully at Dyalog v22.0.

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Comment from xpqz (originally part of #555 :

We should probably use yaml frontmatter for this instead:

```yaml
---
title: Your Page Title
search:
boost: 2
keywords: gets, arrow, assignment operator
---

# Your Page Content
```

I tried this early on but couldn't seem to get it working.

Note to self: this would require changes in the PDF and CHM makers.

The issue names no implementation files or tests. Start by tracing how hidden keywords are represented and included in search extracts, then inspect the PDF and CHM makers and the planned replacement. Done means keywords are supplied through frontmatter, omitted from search-result extracts, and remain compatible with the relevant documentation outputs.

Written by the indexing model from the issue text.