Add documentation for eradicate, flake8-import-conventions, and flake8-no-pep420 (#2652)

docs/rules/commented-out-code.md

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+# commented-out-code (ERA001)
+
+Derived from the **eradicate** linter.
+
+Autofix is always available.
+
+### What it does
+Checks for commented-out Python code.
+
+### Why is this bad?
+Commented-out code is dead code, and is often included inadvertently.
+It should be removed.
+
+### Example
+```python
+```

docs/rules/implicit-namespace-package.md

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+# implicit-namespace-package (INP001)
+
+Derived from the **flake8-no-pep420** linter.
+
+### What it does
+Checks for packages that are missing an `__init__.py` file.
+
+### Why is this bad?
+Python packages are directories that contain a file named `__init__.py`.
+The existence of this file indicates that the directory is a Python
+package, and so it can be imported the same way a module can be
+imported.
+
+Directories that lack an `__init__.py` file can still be imported, but
+they're indicative of a special kind of package, known as a namespace
+package (see: [PEP 420](https://www.python.org/dev/peps/pep-0420/)).
+
+Namespace packages are a relatively new feature of Python, and they're
+not widely used. So a package that lacks an `__init__.py` file is
+typically meant to be a regular package, and the absence of the
+`__init__.py` file is probably an oversight.

docs/rules/unconventional-import-alias.md

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+# unconventional-import-alias (ICN001)
+
+Derived from the **flake8-import-conventions** linter.
+
+### What it does
+Checks for imports that are typically imported using a common convention,
+like `import pandas as pd`, and enforces that convention.
+
+### Why is this bad?
+Consistency is good. Use a common convention for imports to make your code
+more readable and idiomatic.
+
+For example, `import pandas as pd` is a common
+convention for importing the `pandas` library, and users typically expect
+Pandas to be aliased as `pd`.
+
+### Example
+```python
+import pandas
+```
+
+Use instead:
+```python
+import pandas as pd
+```