docs: add SDK development guide, master image guide and extend getting started

[SirHerr]

Summary

Adds task-oriented user documentation for developing analyses and requesting master images, and fills out the Getting Started section.

User guide

• Developing an Analysis with the SDK (guide/user/sdk-how-to.md): pattern vs. Core SDK, then one section each on getting data (FHIR, files from S3), computing on the node, communication between nodes, and results (final, intermediate, aggregation).
• Requesting a Master Image (guide/user/master-images.md): check existing images, create a new one from an example, test it locally, open a pull request, synchronize the Hub.
• Both pages are linked in the sidebar and the user guide overview; the image step in Starting an Analysis links to the master image guide.

Admin guide

• Bucket Setup for Data Store: highlighted note that the S3 user needs both the "List" and the "Read" permission.

Getting started

• Architecture: Hub and Node, lifecycle of an analysis, communication, where data lives.
• Introduction: "FLAME at a Glance" with entry points per role; the security section no longer refers to a flow chart that is not shown.
• Glossary: 27 missing terms added, page title and alphabetical order; sidebar label changed from "Glossar" to "Glossary".

Team

• Hammam Abu Attieh moved to alumni.

Notes for reviewers

• Code and shell snippets were written from the SDK reference and the master-images repository and were not executed. Please check the Core SDK messaging example and the docker commands in particular.
• The master image guide tells contributors to open pull requests against master and to place new images in data/python/use-cases/<name>/.
• The architecture page describes the aggregator as one of the nodes, following the FAQs and SDK docs; the diagram draws it next to the Hub services.
• The existing glossary entries "RSA - key" and "Paillier - key" were left unchanged, but conflict with Security & Operations (ECDH + AES-GCM).

Closes #88
Closes #89
Closes #90
Closes #91
Closes #92
Closes #93

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