[docs-infra] Allows to duplicate markdown

[alexfauquette]

Context

In the package, @mui/x-date-pickers components are not fully independent. Props are shared between multiple components, but not all of them, which makes it difficult to build the documentation. For example, the DatePicker shares a lot of logic with DateTimePickers and with their subcomponent the DayCalendar.

To avoid having to read the entire docs to customize components, we thought about the following strategy:

• Shared properties are deeply discussed in the smallest component using them. For example calendar customization are discussed in a page about DayCalendar
• A brief introduction is added to each page using them.

Problem

Such an approach has a major drawback: creating duplicate content. Which can lead to outdated content.

Solution

The proposed solution is to keep one source of truth for the content we want to repeat.

For example we could add the following in different doc pages

{{"insert": "../../common/pickers/DayCalendar"}}

And define the introduction with

## Calendar customization

The Calendar can be deeply customized by ...

Here is an example of how to ...

{{"demo": ....}}

For more information, have a look at ... page

---

I made an experiment in the Buton page of mui/base documentation

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