[Docs] English introduction

[evdcush]

Motivation

"Introduction" is the entry-point for new users to MMEngine and our documentation.
However, while the Chinese documentation has an introduction, the English documentation lacks an introduction and refers readers to the Chinese docs.

This branch provides an English translation of the Chinese "Introduction" document.

Modification

This branch modifies the English docs/en/get_started/introduction.md by providing a translation from the Chinese version (permalink).

BC-breaking (Optional)

N/A

Use cases (Optional)

N/A

Checklist

1. ✅ Pre-commit or other linting tools are used to fix the potential lint issues.
2. ✅ The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. ✅ If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. ✅ The documentation has been modified accordingly, like docstring or example tutorials.

[zhouzaida]

Hi @evdcush , thanks for your contribution. This PR can be merged after resolving the comments above.

[evdcush]

Hi @evdcush , thanks for your contribution. This PR can be merged after resolving the comments above.

@zhouzaida thank you for your review!
I will push fixes soon.