Add initial xrefsect support

[utzig]

Add basic parsing support and rendering of xrefsect blocks. The current implementation does not generate links or any pages.

An xrefsect is generated when a @xrefitem keyword is found in Doxygen documentation, as well as the specialized forms like \bug, \test and \deprecated.

https://www.doxygen.nl/manual/commands.html#cmdxrefitem

[utzig]

For an example, this source:

/**
 * This is just some non-useful text.
 *
 * @xrefitem examples "Examples" "Examples" "Some xrefitem example"
 *
 * @bug Super buggy function
 */
void some_xref_fn(void);

/**
 * More non-useful text.
 *
 * @xrefitem examples "Examples" "Examples" "Some xrefitem example"
 *
 * @deprecated Don't use this
 */
void another_xref_fn(void);

Will render like this:

[vermeeren]

@utzig Diff itself is good, but could you add the documentation example to the doc? I'm thinking somewhere on documentation/source/specific.rst is good. Probably with a title like Doxygen xrefsect. Thanks!

[utzig]

@utzig Diff itself is good, but could you add the documentation example to the doc? I'm thinking somewhere on documentation/source/specific.rst is good. Probably with a title like Doxygen xrefsect. Thanks!

Done.

[vermeeren]

@utzig Thanks, much appreciated!

[vermeeren]

@utzig Released as v4.23.0.