docs: add developer glossary to getting_started section

Converts the concepts glossary from tutorials/nextcloud-dev-course/ into
a properly formatted RST page at developer_manual/getting_started/glossary.rst.
Adds it as the first entry in the getting_started toctree so new contributors
find it before diving into environment setup.

Improvements over the source document:
- Full RST conversion: code-block directives, list-table, :command: roles
- Added .. contents:: local TOC for easy navigation
- Added .. _csrf: and .. _ocs-api: anchors for cross-referencing
- Replaced broken cross-manual :doc: ref with an external URL
- Expanded several entries for clarity and consistency with docs style
- Removed tutorial-specific environment instructions where not appropriate

AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala