docs: say what mediator, CQRS and event sourcing mean

[yesbert]

What this changes

The landing page, the README and the glossary all used "CQRS" without ever expanding it, and the glossary opened by saying it explains "common CQRS / Event-Sourcing terms" — which assumes the reader already has them. A developer who has not met the patterns has nowhere to land.

• Landing page: a slim three-column strip between the hero and the doors — mediator, CQRS spelled out, event sourcing — two or three sentences each, then on to the glossary and to Why Event Sourcing. Deliberately short and visually quiet, so a reader who knows the terms skims past it in three seconds.
• Glossary: CQRS and Event Sourcing as entries of their own, ahead of the terms that depend on them, and Mediator now names the pattern before naming our type.
• README: one short paragraph expanding the three terms where they are first used.

No deep treatment of the patterns — Fowler and Microsoft Learn own that ground, and a framework's docs competing there read as padding. This is the minimum needed for the page to make sense to someone meeting the words for the first time.

How it was verified

• Local gauntlet green (./scripts/local-gauntlet.sh)
• docfx build --warningsAsErrors — 0 warnings, 0 errors
• Rendered and checked against the doors below it

[Copilot]

🟡 Changes recommended

The glossary still claims it is derived from the session-context capability, which is misleading now that it defines broader concepts like CQRS and Event Sourcing.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR improves the onboarding experience in Stratara’s documentation by briefly defining Mediator, CQRS (spelled out), and Event Sourcing at their first appearance, so readers new to the patterns can understand the rest of the docs.

Changes:

• Adds a short “New to the terms?” paragraph to the top of README.md expanding Mediator/CQRS/Event Sourcing and pointing to the glossary.
• Adds first-class CQRS and Event Sourcing entries to the glossary and clarifies the Mediator entry to name the pattern first.
• Adds a small “basics” strip to the landing page plus CSS styles to support the new layout.

File summaries

File	Description
README.md	Adds brief expansions/definitions for Mediator, CQRS, and Event Sourcing at first mention.
docs/templates/stratara/public/main.css	Introduces styles for the new “basics” section on the landing page.
docs/overview/glossary.md	Adds CQRS/Event Sourcing entries and updates the Mediator definition to reference the pattern.
docs/index.md	Adds a short “thirty-second” explainer strip with links to the glossary and “Why event sourcing”.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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