docs: connect to snap docs for intersphinx

[medubelko]

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Request

The snap docs are now hosted on RTD. Most of our links to snapcraft.io are now broken.

The snap docs don't currently provide an interface for intersphinx. Once they do, we should:

• Connect to them with intersphinx
• Fix all links to be idiomatic external refs

Document location

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[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/SNAPCRAFT-1292.

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