build: improve overview page

[crazy-max]

improves a bit the overview page:

• change title for bake and cache pages to fit with sidebar nav
• missing link to cache page
• missing multi-stage section

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first commit fix an issue with button padding, looks to be a regression from a608900 (cc @thaJeztah):

Before: https://docs.docker.com/desktop/install/mac-install/

Now:

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[dvdksn]

good look! just some small text updates while we're here

[crazy-max]

@dvdksn Thanks for the review! Should be good.