Begin update of Docsy theme to docsy v0.7.0

[deining]

• This PR update the theme, the README file, and the hugo and go versions.
• Contributes to Upgrade Docsy to 0.7.0 #207
• The preview on Netlify doesn't look good, especially the start page. These regressions introduced by the upgrade to bootstrap 5.2.3 should be fixed.

[chalin]

@deining - I've rebased and set the version to 0.7.0-dev... in the NPM package specification since I'm going to merge this now and incrementally handle the migration (just like we did for the main Docsy site). It'll be easier to manage that way.

/cc @LisaFC @geriom