Upgrade to Docsy 0.5.1

[chalin]

• We'll be building upon the successful integration of: #48721
• This is a step of #44002
• Partly contributes to #41171
• Upgrade to 0.5.1 from 0.3.0, because 0.5.1 is the first official version that supports use of Docsy via an NPM module.

Issues found and to be resolved in scope of this work:

• [ ] #32905
• [ ] #48806
• [ ] #48808
• [ ] #48811. Actually, all v3-v5 IDs should be updated to use the v6 icon's canonical name
• [ ] #48813

Note that @sftim did some preliminary exploration:

• 48722

• 48723

Resources:

• Changelog#v0.5.1
• https://github.com/google/docsy/compare/v0.3.0...v0.5.1

/assign /cc @nate-double-u

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