documentation-builder: Add fonts to better handle unicode data from hotdoc-html-2-pdf

It's 2019 and *TeX still lacks proper glyph fallback, so we get the following errors on make pdf on the docs/apertis-docs> repo:

./hotdoc-html-2-pdf build/html/appdev-debugging.html build/pdf/appdev-debugging.pdf --remote-prefix "https://developer.apertis.org" --line-numbers
[WARNING] Missing character: There is no ▸ in font [lmmono10-regular]:!
[WARNING] Missing character: There is no ▸ in font [lmmono10-regular]:!
./hotdoc-html-2-pdf build/html/programming-guide-tooling.html build/pdf/programming-guide-tooling.pdf --remote-prefix "https://developer.apertis.org" --line-numbers
[WARNING] Missing character: There is no 李 in font [lmmono10-regular]:!
[WARNING] Missing character: There is no 健 in font [lmmono10-regular]:!
[WARNING] Missing character: There is no 秋 in font [lmmono10-regular]:!
./hotdoc-html-2-pdf build/html/vm-config.html build/pdf/vm-config.pdf --remote-prefix "https://developer.apertis.org" --line-numbers
[WARNING] Missing character: There is no ▸ in font [lmmono10-regular]:!
./hotdoc-html-2-pdf build/html/vm-management.html build/pdf/vm-management.pdf --remote-prefix "https://developer.apertis.org" --line-numbers
[WARNING] Missing character: There is no ▸ in font [lmmono10-regular]:!
[WARNING] Missing character: There is no ▸ in font [lmmono10-regular]:!
./hotdoc-html-2-pdf build/html/vm-setup.html build/pdf/vm-setup.pdf --remote-prefix "https://developer.apertis.org" --line-numbers
[WARNING] Missing character: There is no ▸ in font [lmmono10-regular]:!
[WARNING] Missing character: There is no ▸ in font [lmmono10-regular]:!

A commit in the documentation repository addresses those by:

• setting up a CJK font so we don't erase team members
• setting up a monospace font which has large Unicode coverage without requiring fallbacks, so a ▸ in a code snippets doesn't end up in a unrendered character

This commits adds the required fonts to the Docker image used for building the documentation.

See https://gitlab.apertis.org/docs/apertis-docs/merge_requests/18 for the actual code change.