I noticed that Markup elements are currently not standardized in any way
- even though the code does make some assumptions.

For example, there are visitors that depend on all elements implementing
`accept`, but no parent class or interface that will help guarantee
every element implements it. Some elements are implemented as structs,
while other are classes. Some have parent classes for shared
functionality and APIs and others don't.

I propose that we standardize the markup elements a bit more, so that we
can have a common interface for them. It will make it easier for the
RDoc to Markdown translator if we can enforce that every markup element
has to be able to print itself in Markdown - and I believe the code will
be easier to understand too.

This PR creates a base `Markup::Element` class and I standardized
`Heading` and `Table` as an example. I'm also finding it challenging to
understand the logic with no clue about what types are and what is
expected from each method, so I also propose we annotate some of these
as documentation - even if we're not adopting type checking.

If folks agree, I will continue to standardize more markup elements. I'm
trying to document what each element represents along the way.

Continuing the work from #1389. This PR standardizes the `HardBreak`
element to a consistent style and starts inheriting from `Element`.

Continuing the work from #1389. This PR standardizes the `BlankLine`
element to a consistent style and starts inheriting from `Element`.

Continuing the work from #1389. This PR standardizes the `Raw` element
to a consistent style and starts inheriting from `Element`.

Add a mention in both contributing and agents about the style of type
annotations we're using to help navigate the codebase and lower the
barrier to new contributors.