docs: make Reference > Glossary the one place terms are defined

[abrichr]

Richard wants key terms such as "qualify" defined in one place that the README, the docs and the CLI help can all link to. This makes Reference > Glossary that place. It builds on the #240 structure and moves no pages.

Changes

• Qualify (#qualify), Certify (#certify) and Certification policy (#certification-policy) replace the old Qualification, Certification and Policy entries. They match the engine's terms page, packages/flow/docs/QUALIFICATION_TERMS.md, which is now on the monorepo's main. That resolves the TODO waiting on openadapt-flow#532. Each entry links to the engine page for the details: the fault cases, what gets signed, and the policy rules.
• The old anchors #qualification, #certification and #policy are kept as <a id> targets, so existing links still land on the right entry.
• New entries: Bundle, Demonstration ("a person or an agent shows the task"; "one or more demonstrations go into the compiler"), and Runner.
• Substrate now leads with vision: OpenAdapt reads every substrate from what is on screen. DOM, UIA, AX or AT-SPI add evidence where a substrate has them, and RDP and Citrix give pixels only.
• The plain-language glossary, the Reference index and llms.txt now point at this page as the canonical definitions. The plain-language page stays a map from plain terms to these entries.
• qualify --certify isn't in a Flow release yet (tests/test_documented_flow_options.py checks documented options against the released CLI), so the Certify entry names it as a main-only option.

Checks

• mkdocs build --strict, scripts/validate_docs.py and pytest tests all pass (546 passed, 1 skipped).
• lychee, offline with --include-fragments, over the built site: 1,146 unique links, 0 errors.

Follow-ups

• The README links qualify here in OpenAdaptAI/OpenAdapt (the README PR).
• Flow's CLI help still points at the relative path docs/QUALIFICATION_TERMS.md, which means nothing to someone who installed from PyPI, and one help string uses a #policies anchor that doesn't exist. That's package code, which the carry-over work is changing right now, so it's left for after the carry-over.