Refactor Markdown classes

[caendesilva]

Refactors Markdown related classes as a lot of these are pretty much as old as the framework itself.

Notable changes:

• Breaking: Rename shared data property $markdown to $content
  
• The SemanticDocumentationArticle class has been heavily refactored, scoping down the public code API
• Page Markdown is now rendered using HtmlStrings ({{ $content }} instead of {!! $markdown !!})
• The MarkdownDocument class is now Htmlable

[codecov]

Codecov Report

Merging #651 (13ea227) into master (2bb888d) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master      #651   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity      1020      1024    +4     
===========================================
  Files            126       126           
  Lines           2544      2551    +7     
===========================================
+ Hits            2544      2551    +7     

Impacted Files	Coverage Δ
...res/Documentation/SemanticDocumentationArticle.php	100.00% <100.00%> (ø)
...ackages/framework/src/Markdown/Models/Markdown.php	100.00% <100.00%> (ø)
.../framework/src/Pages/Concerns/BaseMarkdownPage.php	100.00% <100.00%> (ø)

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[what-the-diff]

• Changed the visibility of Markdown::$body from public to protected.

Click to show WTD guesses that were not quite right

• Added a new method, Markdown::compile(), which compiles markdown into HTML and returns it as a string.

• Renamed the existing compile() method on DocumentationPage to render(). This is because we now have two methods named "compile" in this class: one that renders documentation pages (which was previously called "render") and another that compiles markdowns into HTML strings (the newly added function). The old compile() has been renamed so there's no confusion between these two functions with similar names but different purposes/signatures/return types etc...

• Updated all references to $markdown->body throughout Hyde\Framework\Testing\Feature namespace accordingly; iow, changed them from $page->markdown->body -> (string) $page->markdown. I did not update any other usages of body outside this test suite since they are either already using getters or don't need updating for some reason - eg., if you look at how \Hyde\Pages uses body(), it doesn't actually use its return value anywhere except when calling strlen(); instead, what matters here is just whether or not an empty string gets returned by body(). So even though technically speaking we should be changing those calls too, doing so would add unnecessary complexity without adding much benefit IMO :)

[caendesilva]

This is why I am very happy we have Percy installed. It seems like this has broken parts of the custom Markdown system. Some Markdown parts are not being compiled, it seems to be the custom colored blockquotes and tables, so I think somewhere we lost passing data to the Markdown compiler about the rendered page so we're using the simple Markdown converter instead of the dynamic one.