[Project] Add project instructions and example project.

[mzr1996]

In this PR, we introduce a new projects folder in MMClassification. The projects is used to support some new algorithms and community projects efficiently. In this folder, we won't be strict with code style and other requirements. And some projects can also be moved to the core package after refactoring.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[tonysy]

LGTM