Document handling of content vs. non-content page resources

[jmooring]

In v0.123.0 we handle all content files (.adoc, .html, .md, .org, .pdc, and .rst) the same way. Before v0.123.0 we just copied raw HTML files (files starting with an HTML element that is not a commented out front matter) directly to the public directory without any processing, just as if they were placed in the /static directory.

• In v0.123.0, everything inside /content is either a Page or a bundled Resource.
• Before v0.123.0, we had two cases we treated as "static content": the raw HTML files (see above) and non-content files (e.g. images, text files) that lived in folders below branch bundles. Now that we have lifted that restriction, everything is either a Page or a bundled Resource.
• And all pages and resources will get a logical path.

content/
└── posts/
    └── post-1/
        ├── foo.jpg
        ├── index.md
        ├── with-front-matter.html
        ├── with-front-matter.md
        ├── without-front-matter-begins-with-<.html
        ├── without-front-matter-does-not-begin-with-<.html
        └── without-front-matter.md

v0.122.0

File	Page resource	Copied to public
foo.jpg	✔️	✔️
with-front-matter.html	✔️	❌
with-front-matter.md	✔️	❌
without-front-matter-begins-with-<.html	✔️	✔️
without-front-matter-does-not-begin-with-<.html	✔️	❌
without-front-matter.md	✔️	❌

v0.123.0

File	Page resource	Copied to public
foo.jpg	✔️	✔️
with-front-matter.html	✔️	❌
with-front-matter.md	✔️	❌
without-front-matter-begins-with-<.html	✔️	❌
without-front-matter-does-not-begin-with-<.html	✔️	❌
without-front-matter.md	✔️	❌

To test the above:

git clone --single-branch -b hugo-github-issue-11999 https://github.com/jmooring/hugo-testing hugo-github-issue-11999
cd hugo-github-issue-11999
rm -rf public/ && hugo && tree public

Address terminology issues and clarification requests as described here:
https://discourse.gohugo.io/t/shortcode-for-iframe-breaking-at-update-hugo-122-x-123-8/48904/13

[jmooring]

This was undocumented behavior, but we still need to document the difference between content files and non-content files in a page bundle.

See https://discourse.gohugo.io/t/companion-page-bundle-html-files-not-being-copied-to-subfolder-of-output-directory-anymore/48502/2 for examples and new way to handle this.