Document glossary of Mathesar-specific terminology for new contributors

[seancolsen]

e.g.

• record selector
• table inspector
• table widget
• virtual list
• placeholder row
• exploration
• record summary
• record summary (transitive)
• record summary template
• metadata
• reflection
• service layer
• table link
• UI type
• summarization
• schema
• public (schema)

[kgodey]

We already have a section for this on the wiki: https://wiki.mathesar.org/en/engineering/glossary

[github-actions[bot]]

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