include directive: start from alternative heading level

Thanks for the great project. I perfectly fits the use case I have, with one minor issue.

TL;DR: Is it possible to fix or ignore non-consecutive header levels?

For a group of related projects, I want the contributors to write all the documentation in a single README.md. From the README and the docstrings, I generate the online docs with Sphinx and MyST. I want the sections of the README to end up in separate documents of my generated docs. The docs/ directory is populated with generic stub files that are generated by a cookiecutter.

The README always has the same sections (second level):

# project-name
## Usage
## Installation
## Changelog
### 0.1.0 (2020-10-01)
## Contributing
...

From this, I generate separate pages (e.g. changelog.md):

# Changelog

```{include} ..\README.md
:start-after: "## Changelog"
:end-before: "## Contributing"

My index.md:

# project-name

```{toctree}
installation.md
usage.md
api.md
changelog.md
contribution.md
```

What was a level 2 heading now should be a level 1 heading in the split documents. When building the docs, MyST emits the warning Non-consecutive header level increase because of this. The warning is correct and intended, see #71, but is there any way around this? I want to treat Sphinx warnings as errors, because it has proven very helpful in the past.

Possible solutions:

• Is it possible to disable the specific warning? Sphinx offers suppress_warnings, but I couldn’t figure out the type for this warning.
• Is there a possibility to modify header levels of the included documents, or to set the initial level from 0 to 1?

[chrisjsewell]

With https://github.com/executablebooks/MyST-Parser/pull/313, this warning can now be suppressed.

I will leave this issue open though, as for include it would probably be better to have an extra option to “add/minus x” to the initial heading level

[chrisjsewell]

No this warning arises directly from MyST-Parser: https://github.com/executablebooks/MyST-Parser/blob/4e02629f5fb575fdd733ca43891bebb2e312ad00/myst_parser/docutils_renderer.py#L258-L266

correctly, because for example

# title
## sub-title

# title
### sub-title

## title
### sub-title

all are parsed to sphinx AST:

<section>
  <section>

which will then be always rendered as

# title
## sub-title

so it is not possible to conserve the original levels, if they are not consecutive and starting from 1

include is a special case though, since you are inserting into an existing document, it does make sense to potentially start at a level greater than 1