Release v0.2.2

[haraldng]

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• You ran the local CI checker with ./check.sh with no errors
• You reference which issue is being closed in the PR text (if applicable)
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Issues

Fix #120

Other Changes

This PR includes changes intended for the release of version 0.2.2 on crates. Once it is merged, it will also be published on crates. The changes include:

• Updated code and style of the dashboard.
• Moved the lfu cache implementation as sub module of unicache.
• Add page, links, and other docs related to the playground and dashboard.

Other changes since last release include:

• UniCache feature,
• Dashboard feature,
• Updated macro for defining Snapshot,
• Improved code for storage recovery,