feat(timeline): Content is optional and title mandatory - FRONT-2036

[planctus]

PR description

Please drop a few lines about the PR: what it does, how to test it, etc.

QA Checklist

In order to ensure a safe and quick review, please check that your PR follow those guidelines:

• I have put the vanilla component as devDependencies
• I have put the specs package as devDependencies
• I have added the components directly used in the twig file (with include or embed) as dependencies
• My component is listed in @ecl-twig/ec-components's dependencies
• My variables naming follow the guidelines (snake case for twig)
• I have provided tests
• I have provided documentation (for the "notes" tab)
• If my local yarn.lock contains changes, I have committed it
• I have given my PR the proper label (pr: review needed to indicate that I'm done and now waiting for a review ,pr: wip to indicate that I'm actively working on it ...)

[emeryro]

Won't it be an issue to define the content as a string only?
According to the usage page, users can put other type of content in it (media for instance)

[planctus]

well, i'd say that in these docs on all our templates we refer to the data type, whatever this variable will contain it's going to be used as a string, not an array, not an object, not a boolean, in other words. But you are right that maybe we should treat this, as the other variables that are "allowed" or supposed to contain html (the ones that we were rendering with |raw) in a different way, because string in those cases might be confused with "plain text", we could say something like html markup marked as a safe string to somehow specify that it would be otherwise escaped in an environment with the twig autoescape on.