10539 export.rst doc edit (#10565)
10539 export.rst doc edit (#10565)

* #10539 export.rst doc edit

* Update sphinx/source/docs/user_guide/export.rst

Co-authored-by: Timo Cornelius Metzger <c920ee650506ce38c0734c98dde21c189c0817ef@gmx.net>

* Updated a paragraph on limitations on lines 85-88

* Restructured export.rst

Fixed headings and some minor issues and restructured the doc.

I think that with ``export_svg`` / ``export_svgs`` the sub-section on limitation has no purpose, so I dropped it entirely.

I believe that the underlying docs for the two functions have been copy/pasted rather carelessly and so I'm unsure whether the snippets make sense and/or work. Have a look at [``export_svg``](https://docs.bokeh.org/en/latest/docs/reference/io.html#bokeh.io.export.export_svg) and [``export_svgs``](https://docs.bokeh.org/en/latest/docs/reference/io.html#bokeh.io.export.export_svgs) and let me know if these need to be edited. I think, given that the latter should return a list, the ``filename`` parameter should be different somehow.

Let me know what you think.

* whitespace

* Bullet list fix and minor revision

This changes the bullet lists to use asterisks and revises the 'Additional dependencies' section.

* Add missing Sphinx substitution

Co-authored-by: Timo Cornelius Metzger <c920ee650506ce38c0734c98dde21c189c0817ef@gmx.net>
Co-authored-by: Bryan Van de Ven <0a3ba1af216659500dd8f41e29f3979f42452870@bokeh.org>
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