docs: add serialization docs, update chunking docs (#1556)

* docs: add serializers docs, update chunking docs

Signed-off-by: Panos Vagenas <pva@zurich.ibm.com>

* update notebook to improve MD table rendering

Signed-off-by: Panos Vagenas <pva@zurich.ibm.com>

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Signed-off-by: Panos Vagenas <pva@zurich.ibm.com>

docs/concepts/serialization.md

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+## Introduction
+
+A *document serializer* (AKA simply *serializer*) is a Docling abstraction that is
+initialized with a given [`DoclingDocument`](./docling_document.md) and returns a
+textual representation for that document.
+
+Besides the document serializer, Docling defines similar abstractions for several
+document subcomponents, for example: *text serializer*, *table serializer*,
+*picture serializer*, *list serializer*, *inline serializer*, and more.
+
+Last but not least, a *serializer provider* is a wrapper that abstracts the
+document serialization strategy from the document instance.
+
+## Base classes
+
+To enable both flexibility for downstream applications and out-of-the-box utility,
+Docling defines a serialization class hierarchy, providing:
+
+- base types for the above abstractions: `BaseDocSerializer`, as well as
+  `BaseTextSerializer`, `BaseTableSerializer` etc, and `BaseSerializerProvider`, and
+- specific subclasses for the above-mentioned base types, e.g. `MarkdownDocSerializer`.
+
+You can review all methods required to define the above base classes [here](https://github.com/docling-project/docling-core/blob/main/docling_core/transforms/serializer/base.py).
+
+From a client perspective, the most relevant is `BaseDocSerializer.serialize()`, which
+returns the textual representation, as well as relevant metadata on which document
+components contributed to that serialization.
+
+## Use in `DoclingDocument` export methods
+
+Docling provides predefined serializers for Markdown, HTML, and DocTags.
+
+The respective `DoclingDocument` export methods (e.g. `export_to_markdown()`) are
+provided as user shorthands — internally directly instantiating and delegating to
+respective serializers.
+
+## Examples
+
+For an example showcasing how to use serializers, see
+[here](../examples/serialization.ipynb).