GI-DocGen 2021.7

Changes since 2021.6:

• Add a "check" sub-command [#51]
• Allow defining multiple content directories
• Detect gtk-doc code blocks for JavaScript
• Multiple changes to the basic template
• Add in-page content navigation
• Filter hidden data from indices
• Allow defining hidden symbols matching a regular expression
• Handle (attribute element-type) annotation for GListModel
• Fix class method linking [#92]
• Fix links to callback type arguments
• Fix argument listing in class methods
• Don't grab the 's' key when the sidebar is hidden [#97]
• Index the extra content files for search [#64]
• Fix gtk-doc sigil handling in code blocks
• Re-instate gtk-doc () function detection