Add Core Concepts and Modules Overview sections to usage guide

[vlovgr]

• Add Core Concepts and Modules Overview sections to usage guide.
• Change headings in the documentation to be hyperlinks with anchors.
• Change links in the usage guide to be underlined when hovered.
  Also use text-decoration-skip: ink; throughout microsite.
• The ConfigValue class now has a sensible toString method.

[codecov]

Codecov Report

Merging #19 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #19   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          23     23           
  Lines         335    336    +1     
  Branches        0      1    +1     
=====================================
+ Hits          335    336    +1

Impacted Files	Coverage Δ
...core/shared/src/main/scala/ciris/ConfigValue.scala	100% <100%> (ø)	⬆️

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