Add pdf-specific rendering and profiles

[ianalexmac]

This pull request adds separate Quarto Profiles for the HTML build and the PDF build. Doing so allows us to make format-specific changes, such as omitting data.qmd from the pdf build (no need for a data portal in print format). This represents a major step forward in the control of our renderings while maintaining our streamlined single-job workflow.

Changes overview:

• _quarto.yml has been split into two profiles: _quarto-html.yml and _quarto-pdf.yml

• _quarto.yml is now just a pointer towards the two profiles

• html render and publish to GitHub Pages remains unchanged, all chapters included
• pdf render has content restricted to index.html and about.html for demonstration purposes. View the pdf here

Going forward, we can now modify content rendered to the PDF, which keeps us all sane.

Note: commits were squashed, see fork for original commit history.

[eldobbins]

Clever solution to a problem that has been nagging me.

[eldobbins]

@ianalexmac did you document how you figured this out? If so, please link to your trail of breadcrumbs. Otherwise, write something up in the wiki.