Split documentation to multiple files in docs/ and use markdown

docs/internals/cache.md

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+# Cache
+
+By default, the linter uses a cache to prevent linting the same migration multiple times.
+The default location of the cache on Linux is
+`/home/<username>/.cache/django-migration-linter/<version>/<ldjango-project>_<database_name>.pickle`.
+
+Since the linter uses hashes of the file's content, modifying a migration file will re-run the linter on that migration.
+If you want to run the linter without cache, use the flag `--no-cache`.
+If you want to invalidate the cache, delete the cache folder.
+The cache folder can also be defined manually through the `--cache-path` option.