Add detailed description for two diagrams

[peacekeeper]

This adds long descriptions for additional two diagrams:

• https://www.w3.org/TR/did-core/#resolve-resolverepresentation
• https://www.w3.org/TR/did-core/#did-url-dereference-overview

This uses the same approach as in https://github.com/w3c/did-core/pull/781, and partially re-uses @clehner 's content from that PR for a similar diagram here.

Partially addresses https://github.com/w3c/did-core/issues/625.

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[peacekeeper]

@TallTed and @clehner I applied all your suggestions.

[msporny]

Editorial, multiple reviews, changes requested and made, no objections, merging.