feat: add back missing dashboard info and minor content improvements

[guolau]

Summary of changes

• Added back missing info
• Replaced AI-gen CSS examples with our previous docs' examples
• Fix Incidents page not listed in sidebar
• Merged Customization page with Configuration page (I think this makes more sense, so the config options aren't scattered)
• Created a separate page for Custom CSS (since we have long code samples there)
• Other misc changes
  • Replace some screenshots with more polished ones
  • Tweaked wording to better reflect Checkly vernacular
  • Updated the "only available on X plan" callouts to be more generic ("only available on specific plans"), so we have one source of truth (i.e. Pricing page)
  • Removed some generic/unverified AI-gen content

[guolau]

Since I created/removed a few files, we need these redirects. How can I implement these?

• New docs /communicate/dashboards/customization -> New docs /communicate/dashboards/configuration
• Previous docs /docs/dashboards/dashboard-customization -> New docs /communicate/dashboards/configuration
• Previous docs /docs/dashboards/custom-css/ -> New docs /communicate/dashboards/custom-css

[Dgiordano33]

Looks good, will set up redirects now