Docs: First batch of explanation, tutorials, and how-tos

[dwhswenson]

• Explanation: Core architecture and modules
• Explanation: Lambda modules
• Tutorial: Your first LambdaCron
• How-To: Write a Lambda for LambdaCron
• How-To: Set up SES
• How-To: Use Email Notifier

I consider this to be the bare minimum we need for the docs; that's the goal for this PR.

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EDIT: Holding off on the tutorial until we have a template repo set up for that.

[Copilot]

Pull request overview

This PR sets up an initial MkDocs + Read the Docs documentation site for LambdaCron and adds the first batch of explanation and how-to content, along with diagrams and supporting doc build dependencies.

Changes:

• Add MkDocs configuration (Material theme) and Read the Docs build config.
• Add initial docs pages for architecture/data flow/lambda image modules and operational how-tos (SES, email notifications, test URL, writing lambdas/templates).
• Update dev dependencies/lockfile to include MkDocs (pinned <2) and mkdocs-material.

Reviewed changes

Copilot reviewed 14 out of 18 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pyproject.toml	Add MkDocs + Material theme to dev extras.
pixi.lock	Lockfile updates to include MkDocs/Material and transitive deps.
modules/email-notification/setting-up-ses.md	Remove old SES setup doc (replaced by new docs site content).
mkdocs.yml	New MkDocs site configuration and navigation.
docs/index.md	New docs homepage/landing page.
docs/image-dev/AWSArchitecture.drawio	Add editable source diagrams (draw.io).
docs/how-to/write-lambda-and-templates.md	New how-to for authoring tasks + Jinja templates.
docs/how-to/use-test-url.md	New how-to for using the Lambda Function URL for testing.
docs/how-to/use-email-notifications.md	New how-to for wiring SES email notifications module.
docs/how-to/set-up-ses.md	New SES prerequisite/setup checklist page.
docs/explanation/lambda-architecture.md	New explanation of Lambda container image module patterns.
docs/explanation/data-flow.md	New explanation of _perform_task → SNS/SQS → template rendering data flow.
docs/explanation/architecture.md	New high-level architecture explanation with diagrams.
docs/explanation/AWSArchitecture-ModuleArchitecture.drawio.svg	Add generated diagram asset used by explanation pages.
AGENTS.md	Document Diataxis + MkDocs conventions for future doc contributions.
.readthedocs.yaml	Configure RTD to build MkDocs site from mkdocs.yml.

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[ethanholz]

This looks good, my only criticism is hosting on RTD instead of on our own domain. That opinion is loosely held so no use blocking this PR. Merging.