support nested admonitions

[lebalz]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

When nesting admonitions, the first seen stop-word (Line starting with :::) exits the first admonition block.

Reproducible demo

No response

Steps to reproduce

1. Add the following snippet with nested admonitions to a markdown page

:::info Weather
On nice days, you can enjoy skiing in the mountains.

:::danger Storms
Take care of snowstorms...
:::

:::

Expected behavior

Actual behavior

Your environment

• Docusaurus version used: 2.2.0

Self-service

• I'd be willing to fix this bug myself.