Support for expressive layouts with page.md

[crphang]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] New feature

Resolves #631

Currently this is a WIP but the Proof of Concept generally working. The idea is for user to define a main page structure to allow users to define wrapper content around their main content.

For instance:

Code:

Result:

Definitely a lot more work to be done:

• Because page need to be pre-processed (prior to parser), we have yet to process frontmatter which is required to pass in layout information. Figure out a way to process/wrap page after parser.
• I currently hacked into Parser to make this work. Refactor some part of parser to take in data instead of file.
• Layouts are currently not refreshed during hot reloading.

[yamgent]

No comments so far for the current features that the prototype has, looking forward to the final implementation. 👍

Btw, if there are a lot of refactoring to be done, and those refactoring might also benefit other features (not necessary just for expressive layout), you might want to consider handling the refactoring in separate PRs, so that this PR doesn't get too big.

[crphang]

Keeping things super simple for now to ensure that this PR does not go out of hand:

• Fix problem with parsing and undoing refactor
• User Guide update
• Update test

Important functional and non-functional requirements that this PR takes care of:

1. Allowing user to define any layout style and embed main body content - Core Feature
2. Backward compatibility of existing users if they upgrade.

Many more work to be done but will be separated to different PRs. A non-exhaustive list include:

1. Refactoring layouts logic from insertion of header, footer, ...
2. Removing inference of layouts from frontmatter. Currently the code takes frontmatter as source of truth instead of the Page property. This needs to be updated but it is not that simple.
3. Expressive Layouts Validation. If users change the keyword of expressive layout from MAIN_CONTENT_BODY to something else, pages will not work. We should have validation (for templates created by developers) and soft failure (for user input changes) to skip expressive layouts if user make wrong changes.

[yamgent]

Just to let you know that I will take a look at this soon!

[yamgent]

Appreciate your patience. I left some comments, but I feel that the comment with the '⭐⭐' as the first line is probably the most important thing that you might want to address first.

[crphang]

Ready for review

[damithc]

Reads better this way?

The page.md file of a layout should come with the reserved variable {{ MAIN_CONTENT_BODY }} which injects the actual page content in every page. This allows you to build layouts in different ways.

Alternatively, can be written this way (i.e., avoid breaking sentences across boxes):

The page.md file of a layout should come with the following reserved variable which injects the actual page content in every page.

{{ MAIN_CONTENT_BODY }}

This allows you to build layouts in different ways.