Description of the "Extension" concept and the other concepts

[ghost]

It might be unclear to anyone new to ERC, how the extensions work and what is left, if no extension is used.
The spec documentation should be updated to include a paragraph on this ("the extension concept") and reflect in its structure what is "base" and what is "extension".

[ghost]

Also "design concept" should not be part of the spec, but could form a new category where "the extension concept" might fit in.

[nuest]

Regarding "design concept" and clarification of extensions

• [ ] "Fundamental design concepts" should go into the dev guide, the spec is just facts and no arguing
• [ ] "Extensions" must be a top level section before "Nested runtime", extensions can do what they want but not break the core spec. Which extension is used in a specific ERC is visible by the required configured (as defined in the extension) being present within the ERC configuration file. (Metadata is still left in "ERC configuration file/Extension metada".)
• [ ] Add "extension" and "core" to the glossary.
• [ ] remove "core" from the core specification text (extensions may use the word to clarify...)

[nuest]

Extensions were already improved by #20

[ghost]

Extension concept will be dropped with current spec

[nuest]

See #36