[DOCS] Copy glossary content from Stack Docs glossary

[jrodewig]

This PR copies content from the Stack Docs glossary.

It also adds a few Elasticsearch definitions missing from the original Stack Docs glossary:

• Cross-cluster search
• Filter
• Query

Why make this change?

The end goal is to use this file as a single source for both the Elasticsearch and Stack Docs glossaries.

After this PR is merged, I'll open separate PRs to:

1. Update the Elasticsearch glossary to include terms from this file.
2. Update the Stack Docs glossary to include terms from this file.

See elastic/elasticsearch#40560 for more discussion.

[nik9000]

I'm not a fan of putting this in the docs repo because we don't have proper up-to-date checking on it. Also, it feels like content instead of helpers which feels weird to have in the docs repo. I admit I've not really thought out my last point, but the up-to-date checking still makes this a less good idea than it looks on the face of it.

[jrodewig]

@nik9000 I think up-to-date checking is a big and valid concern. Added the team-discuss label so we can talk about this more before moving forward.

I'm happy to close this PR. At the very least, it revealed some definitions that weren't synced which I can fix in a separate PR if we go another route.

[jrodewig]

We discussed this internally and decide to go with a many-to-one approach (individual solution repos feeding into a combine glossary).

Closing this PR.