[Docs] Move development related docs to owncloud.dev/clients

[michaelstingl]

How?

• Needs folder to store this in client repo, for example:
  https://github.com/owncloud/client/tree/master/docs/developement
• Markdown format
• Can be added to https://owncloud.dev/clients/ (compare web/docs)

What?

• building.adoc
• architecture.adoc
• glossary.adoc
• Resources from other repos
• Update README.md
• guitest.adoc

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