Add Goldmark as the new default markdown handler

This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.

If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:

```toml
[markup]
defaultMarkdownHandler="blackfriday"
```

Fixes #5963
Fixes #1778
Fixes #6355

docs/content/en/getting-started/configuration-markup.md

@@ -0,0 +1,73 @@
+---
+title: Configure Markup
+description: How to handle Markdown and other markup related configuration.
+date: 2019-11-15
+categories: [getting started,fundamentals]
+keywords: [configuration,highlighting]
+weight: 65
+sections_weight: 65
+slug: configuration-markup
+toc: true
+---
+
+## Configure Markup
+
+{{< new-in "0.60.0" >}}
+
+See [Goldmark](#goldmark) for settings related to the default Markdown handler in Hugo.
+
+Below are all markup related configuration in Hugo with their default settings:
+
+{{< code-toggle config="markup" />}}
+
+**See each section below for details.**
+
+### Goldmark
+
+[Goldmark](https://github.com/yuin/goldmark/) is from Hugo 0.60 the default library used for Markdown. It's fast, it's [CommonMark](https://spec.commonmark.org/0.29/) compliant and it's very flexible. Note that the feature set of Goldmark vs Blackfriday isn't the same; you gain a lot but also lose some, but we will work to bridge any gap in the upcoming Hugo versions.
+
+This is the default configuration:
+
+{{< code-toggle config="markup.goldmark" />}}
+
+Some settings explained:
+
+unsafe
+: By default, Goldmark does not render raw HTMLs and potentially dangerous links. If you have lots of inline HTML and/or JavaScript, you may need to turn this on.
+
+typographer
+: This extension substitutes punctuations with typographic entities like [smartypants](https://daringfireball.net/projects/smartypants/).
+
+### Blackfriday
+
+
+[Blackfriday](https://github.com/russross/blackfriday) was Hugo's default Markdown rendering engine, now replaced with Goldmark. But you can still use it: Just set `defaultMarkdownHandler` to `blackfriday` in your top level `markup` config.
+
+This is the default config:
+
+{{< code-toggle config="markup.blackFriday" />}}
+
+### Highlight
+
+This is the default `highlight` configuration. Note that some of these settings can be set per code block, see [Syntax Highlighting](/content-management/syntax-highlighting/).
+
+{{< code-toggle config="markup.highlight" />}}
+
+For `style`, see these galleries:
+
+* [Short snippets](https://xyproto.github.io/splash/docs/all.html)
+* [Long snippets](https://xyproto.github.io/splash/docs/longer/all.html)
+
+For CSS, see [Generate Syntax Highlighter CSS](/content-management/syntax-highlighting/#generate-syntax-highlighter-css).
+
+### Table Of Contents
+
+{{< code-toggle config="markup.tableOfContents" />}}
+
+These settings only works for the Goldmark renderer:
+
+startLevel
+: The heading level, values starting at 1 (`h1`), to start render the table of contents.
+
+endLevel
+: The heading level, inclusive, to stop render the table of contents.