Add support for SECTION GTK-Doc identifier

I'm relatively new with GTK, Glib, etc... and I'm heavily relying on the documentation to understand how I should use all these libraries.

Thus, I'm not sure if the SECTION GTK-Doc identifier is part of GObject Introspection too.

If that's a part, it could be useful for gi-docgen to support this identifier too, because it can contain general information to better understand how to use a tool.

Otherwise, I think we should update libraries documentation to let the current SECTION description to be usable by the gi-docgen.

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For example, I've been looking for general concept of a GString.

In the old developer website (based on gtk-doc), there's a Description containing these information:

A GString is an object that handles the memory management of a C string for you. The emphasis of GString is on text, typically UTF-8. Crucially, the "str" member of a GString is guaranteed to have a trailing nul character, and it is therefore always safe to call functions such as strchr() or g_strdup() on it.

However, a GString can also hold arbitrary binary data, because it has a "len" member, which includes any possible embedded nul characters in the data. Conceptually then, GString is like a GByteArray with the addition of many convenience methods for text, and a guaranteed nul terminator.

They are really useful for newcomers like me, I've really better understood why I should use GString instead of char*.

On the new website (based on gi-docgen), the only information in Description is:

The GString struct contains the public fields of a GString.

which is true, but almost useless compared to gtk-doc 😅