Expand the text in the init timeout message to give the reader more hints as to what may be causing the issue.
The current error sent me on a wild goose chase trying to debug my shortcodes, but it turned out that the issue was just that I wasn't caching my resources/ directory on my build server. Once I found this blog post, I solved the issue in minutes:
https://www.henriksommerfeld.se/hugo-timeout-not-a-circular-loop-in-shortcode/
Googling the error message yields several additional posts from users who focus on loops in their shortcode when the problem seems to be overly-expensive builds.
The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue #5074, "pages from other data sources".
But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.
Most notable changes:
* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
This improves the taxonomy template `.Title` situation and make common template constructs much simpler.
See #5074Fixes#5763Fixes#5758Fixes#5090Fixes#5204Fixes#4695Fixes#5607Fixes#5707Fixes#5719Fixes#3113Fixes#5706Fixes#5767Fixes#5723Fixes#5769Fixes#5770Fixes#5771Fixes#5759Fixes#5776Fixes#5777Fixes#5778
