Navbar-brand: define classes for the logo and name

[chalin]

• Closes #1025
• Closes #1189 by superseding it
• Documents how to hide the navbar brand name
• (Edit) Flips the default: a logo is displayed by default (since that is what most sites need). To hide the logo, set navbar_logo to false (as was already documented in the User Guide).

Preview of the relevant doc sections:

• https://deploy-preview-1190--docsydocs.netlify.app/docs/adding-content/iconsimages/#add-your-logo
• https://deploy-preview-1190--docsydocs.netlify.app/docs/adding-content/lookandfeel/#styling-your-brand-logo-and-name

[chalin]

Since the user guide, as it was written before, seemed to suggest that the Docsy default was to display a logo in the navbar, I fixed the layout code so as to conform to this behavior, and added a note to the breaking-change section of the CHANGELOG.

WDYT @LisaFC @emckean?

[LisaFC]

This looks great, thanks Patrice!