Cleanup and refactor, in preparation for plugin support. (#318)

* Work started moving converters to individual files.
* Significant cleanup and refactor.
* Moved everything to a packages subfolder.
* Added sample plugin.
* Added instructions to the README.md
* Bumped version, and added a note about compatibility.

.github/workflows/tests.yml

@@ -12,14 +12,7 @@ jobs:
             3.10
             3.11
             3.12
-      - name: Set up pip cache
-        if: runner.os == 'Linux'
-        uses: actions/cache@v4
-        with:
-          path: ~/.cache/pip
-          key: ${{ runner.os }}-pip-${{ hashFiles('pyproject.toml') }}
-          restore-keys: ${{ runner.os }}-pip-
       - name: Install Hatch
         run: pipx install hatch
       - name: Run tests
-        run: hatch test
+        run: cd packages/markitdown; hatch test