[Github Actions] Setup PDF build Github Action for QuantEcon

[mmcky]

• maybe we should build an action based on this github action that adds an anaconda environment to the docker and then builds make pdf

[mmcky]

Note: we currently have pdf support via custom config files that do the job so this is about improvements we could do in the future to simplify deployments for new projects

[mmcky]

Things are working pretty well -- but we can join forces with Executable Books on this as part of the mystjs project

[mmcky]

PDF builds have run in-CI for years across the lecture repos, and the reusable-action version this proposed now exists as build-lectures in quantecon/actions (builder: pdflatex, with html-copy-pdf staging). Rollout is tracked in #327 Track 2, and the mystmd path in #161. Closing this 2020 placeholder in their favour.