Support HTML in the header of Site.Params.intro

[gavinwray]

Description

Allow users to put HTML in the intro parameters' header in the sidebar. While config.toml states 'HTML is supported within the params' (line 88), safeHTML is not supported for the header in the sidebar.html partial. It is only enabled for the parameters paragraph and about.

How Has This Been Tested?

1. Edited sidebar.html.
2. Checked that HTML rendered correctly in the sidebar's intro section.

Hugo Version: v0.39

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation, including theme.toml, accordingly.
• I have read the Contributing Document.