fix(theme-docs): remove redundant header

[NozomuIkuta]

Types of changes

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

Description

It is explained in documentation that category in front-matter is optional. However, theme docs renders the category header in AppNav component regardless of existence of it.

• If category is completely not defined in front-matter, theme docs renders undefined as a uppercase text.
• If category is set as an empty string in front-matter, theme docs renders an empty p element with margin-bottom.

Reproduction Link (CodeSandbox)

With my PR, category in front-matter is validated in content:file:beforeInsert hook, and is set as an empty string if it's falsy or not a string. Coercion to an empty string is for its type to match that described in documentation.

In addition, if category is an empty string (i.e. falsy), now AppNav component doesn't render the category header (the p element).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

I tested my changes in local by yarn link the modified package.

[benjamincanac]

Thanks @NozomuIkuta 🙂