Cover `amaranth.build.dsl` with documentation

[Lunaphied]

Starting to document the amaranth.build module and thought the DSL portion was a useful place to start since it's the most frequently encountered.

Due to Git issues I accidentally forced #696 to not exist so here's the fixed PR.

[codecov-commenter]

Codecov Report

Merging #698 (fc12a0b) into main (8b85afa) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #698   +/-   ##
=======================================
  Coverage   81.09%   81.09%           
=======================================
  Files          49       49           
  Lines        6479     6479           
  Branches     1530     1530           
=======================================
  Hits         5254     5254           
  Misses       1032     1032           
  Partials      193      193           

Impacted Files	Coverage Δ
amaranth/build/dsl.py	96.22% <ø> (ø)
amaranth/lib/io.py	73.68% <ø> (ø)

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[whitequark]

Thank you for this contribution. The build DSL is something that's widely considered flawed and we plan to (hopefully) replace it in short to medium term future, so there aren't current plans to write and update documentation for it.