docs: description of supported formats and backends (#788)

* chore: remove type-ignore marks for attaching text to non GroupItems

After commit b74208 of docling-core, text items can be attached to any NodeItem
and therefore the ignore[arg-type] type marks can be removed.

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

* test: remove unnecessary imports

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

* docs: add documentation on supported formats and backends

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

* docs: add notebook example with XML backends

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

---------

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

docs/usage.md

@@ -24,6 +24,20 @@ docling https://arxiv.org/pdf/2206.01062
 
 To see all available options (export formats etc.) run `docling --help`. More details in the [CLI reference page](./reference/cli.md).
 
+### Supported formats
+
+The document conversion in Docling supports several popular formats, including:
+
+- **PDF** (Portable Document Format): the format developed by Adobe to present documents compatible across application software, hardware, and operating systems.
+- **.docx**, **.xlsx**, **.pptx** (Word, Excel, and PowerPoint): the Open XML formats suppored by Microsof Office.
+- **Markdown**:  a lightweight markup language to add formatting elements to plain text documents.
+- **AsciiDoc**: a plain text markup language for writing technical content.
+- **HTML** (Hypertext Markup Language): the standard markup language for creating web pages.
+- **XHTML** (Extensible Hypertext Markup Language): the XML-based version of HTML.
+- **XML** (Extensible Markup Language): a markup format for storing and transmitting data. Due to its flexibility, Docling requires custom implementations to identify the
+semantics of the data. Currently, Docling supports the parsing of [USPTO](https://www.uspto.gov/patents) patents and [PubMed Central® (PMC)](https://pmc.ncbi.nlm.nih.gov/) articles.
+
+
 ### Advanced options
 
 #### Adjust pipeline features
@@ -126,6 +140,32 @@ result = converter.convert(source)
 You can limit the CPU threads used by Docling by setting the environment variable `OMP_NUM_THREADS` accordingly. The default setting is using 4 CPU threads.
 
 
+#### Use specific backend converters
+
+By default, Docling will try to identify the document format to apply the appropriate conversion backend (see the list of [supported formats](#supported-formats)).
+You can restrict the `DocumentConverter` to a set of allowed document formats, as shown in the [Multi-format conversion](./examples/run_with_formats.py) example.
+Alternatively, you can also use the specific backend that matches your document content. For instance, you can use `HTMLDocumentBackend` for HTML pages:
+
+```python
+import urllib.request
+from io import BytesIO
+from docling.backend.html_backend import HTMLDocumentBackend
+from docling.datamodel.base_models import InputFormat
+from docling.datamodel.document import InputDocument
+
+url = "https://en.wikipedia.org/wiki/Duck"
+text = urllib.request.urlopen(url).read()
+in_doc = InputDocument(
+    path_or_stream=BytesIO(text),
+    format=InputFormat.HTML,
+    backend=HTMLDocumentBackend,
+    filename="duck.html",
+)
+backend = HTMLDocumentBackend(in_doc=in_doc, path_or_stream=BytesIO(text))
+result = backend.convert()
+print(result.export_to_markdown())
+```
+
 ## Chunking
 
 You can chunk a Docling document using a [chunker](concepts/chunking.md), such as a