Adds alphabetical links to Glossary

[lcawl]

Depends on elastic/kibana#69721 and elastic/docs#1875

This PR adds alphabetical sections and links to the Glossary (https://www.elastic.co/guide/en/elastic-stack-glossary/current/index.html). It also moves the "Introduction" page into the "Terminology" page, so that there's only one page in this book.

Preview

https://stack-docs_1214.docs-preview.app.elstc.co/guide/en/elastic-stack-glossary/master/terms.html

[gchaps]

The A, B, C headings are showing up in the right side bar.

Also, how about removing the heading "Introduction"? I'm not sure it's needed.

[lcawl]

The A, B, C headings are showing up in the right side bar.

Yes, unfortunately discrete attributes don't seem to affect H3 titles in that "On this page" section after all.

Also, how about removing the heading "Introduction"? I'm not sure it's needed.

I kept it in case there were links to that existing page. I can try removing it and see if anything breaks, however.

[lcawl]

I will revert to sections instead of description lists since I don't like the indentation issues we're hitting per elastic/docs#1876

[gtback]

I will revert to sections instead of description lists

I feel like using section headers for each letter would be more semantically correct than description links anyway. 👍

I imagine there's a bug in how Asciidoctor tries to infer the list hierarchy, because it seems like it would be pretty complex.