Designs at PLANET-1766

Dev tasks:

1. We need to implement a new block in the plugin-blocks
2. Name of the block: Submenu
3. Data: It should be generated automatically from the H tags in the content of the page.

For example, if the editor adds a <h1> A first text here </h1>  and a <h2> A second text here </h2> , the block would create:

- A first text here

- A second text here

With navigation towards these.