feat: Rendering when there are not enough placeholders

[Songmu]

If there are not enough placeholders for title, subtitle, and body for the content, this information will not be displayed.

This is a current specification, but it is unfriendly to users. It may be better to display a warning or render it somewhere.

For example, if the following Markdown is applied to the “TITLE_AND_BODY” style without subtitles instead of a complicated style, users may want the content other than the title to be merged into the body.

input

# CAP theorem

## Consistency

Every read receives the most recent write or an error.

## Availability

Every request received by a non-failing node in the system must result in a response.

## Partition tolerance

The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes.

desired output

Currently, we can change the heading to h3 to display it this way, but it seems strange in terms of information structure, and I feel that it is a workaround.

[Songmu]

I think this issue should be resolved at the same time as #278.

[k1LoW]

I would suggest that a warning should be displayed.

If we add a behavior for when the Placeholder is not present, it is likely that people will use it.

That is not how I envision it being used.

( On the contrary, your idea of “use Image placeholder if available” was a great one 👍 )

[Songmu]

The current layout rules are a bit complicated and may confuse new users. That was the case for me.

It is stressful for users when the content they wrote in Markdown does not appear on the slide due to the current layout rules. It can be challenging to create slides properly without understanding the current layout rules.

Therefore, it would be better to develop a specification that allows the content to be displayed without compromising the current specifications and user experience. I can come up with the specifications.