Document the new "Frontity Slot Block" plugin and "slot-block" processor

[luisherranz]

• Final Implementation: https://community.frontity.org/t/frontity-slot-block/4534/4?u=luisherranz
• WordPress plugin: https://github.com/frontity/frontity-slot-block
• Processor PR: Processor for the Frontity Slot Block plugin frontity#815

I was also wondering if it would be nice to have a Slot & Fill section in the Learning Frontity docs that explain the pattern. After all, it's one of the "core patterns". Perhaps the least known.

Also, maybe we should include some <Slot> components in mars-theme, so beginners know they exist. For example, we could add them before and after the post:

<Content>
  <!-- Add a Slot so other packages can add content here. -->
  <!-- https://docs.frontity.org/learning-frontity/slot-and-fills -->
  <Slot name="Before the post" />

  <!-- Use Html2React to process the HTML generated by WordPress -->
  <!-- https://api.frontity.org/frontity-packages/features-packages/html2react -->
  <Html2React html={post.content.rendered} />

  <!-- Add another Slot after the content. -->
  <Slot name="After the post" />
</Content>

What do you think?

[luisherranz]

By the way, @kasperaamodt has suggested adding also a Fill block, to be able to create fills directly from Gutenberg.

I am going to investigate if it would be possible, but please take into consideration that maybe this becomes the "Frontity Slot and Fill blocks" plugin and there will be a second processor called fill-block as part of this 🙂