build: Allow arbitrary line length in markdown documents (#1250)

**What problem does this PR solve?**:
Maintaining a maximum line length is not necessary.

We view Markdown documents using clients that can (and do) automatically
wrap text to the width of the display.

Moreover, when we render Markdown to HTML using Hugo, the most important
use case for most of Markdown documentation, the line wrapping is
discarded.

We already disabled line length for code blocks. This PR just extends it
to text.

**Which issue(s) this PR fixes**:
Fixes #

**How Has This Been Tested?**:
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Author: dlipovetsky

.markdownlint.json

@@ -1,6 +1,6 @@
 {
   "heading-style": { "style": "atx" },
   "ul-style": { "style": "dash" },
-  "line-length": { "line_length": 120, "code_blocks": false },
+  "line-length": false,
   "hr-style": { "style": "---" }
 }