Collapse adjacent blank lines. (#60)

Most of these runs of blank lines were added as a byproduct of the
RST-to-markdown export. In a few places, these were an intentional part
of authoring — for example, most of the reference has two blank lines
before a section heading — which we could put back if desired. I think
there is an argument to be made for the consistency of single blank
lines, from an authoring perspective. I don't remember the full
rationale from 2014 when Brian and I used the double blanks in the
reference.

Verified changes are only in blank lines with `git diff
--ignore-blank-lines`, which has no output. Also confirmed that there
are no changes in the HTML output from DocC.

Author: amartini51

Sources/TSPL/TSPL.docc/GuidedTour/AboutSwift.md

@@ -1,5 +1,3 @@
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 # About Swift
 
 Understand the high-level goals of the language.
@@ -50,7 +48,6 @@ and it continues to evolve with new features and capabilities.
 Our goals for Swift are ambitious.
 We can’t wait to see what you create with it.
 
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 This source file is part of the Swift.org open source project
 

Sources/TSPL/TSPL.docc/GuidedTour/Compatibility.md

@@ -1,5 +1,3 @@
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 # Version Compatibility
 
 Learn what functionality is available in older language modes.
@@ -51,7 +49,6 @@ that's divided into multiple frameworks,
 you can migrate your code from Swift 4 to Swift 5.7
 one framework at a time.
 
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 This source file is part of the Swift.org open source project