feat(docs): add Glossary, Semantic Variants

[franzheidl]

• Add a first version of the glossary (no links yet as docs structure is still in flux, but to begin collecting keywords)
• Add Semantic Variants page (planned to add these to the Glossary, but the concept seems so fundamental decided it warrants its own page)
• Add a note using SignInForm is mandatory for Sign-In pages.

Closes #1545

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