Docs: contribution_process.svg fails epub validation

[Mike Edmunds]

The contribution_process.svg image causes an ​EPUBCheck error RSC-005 in the docs epub build, due to prohibited <font> tags inside foreignObjects.

This will likely prevent that image from appearing correctly in some eReaders. (Though it shouldn't prevent the epub as a whole from rendering.)

Also, a large part of that svg file is some ~20KB of draw.io original editable source, serialized into in a nonstandard content attribute. It might be helpful to keep the editable source file separate from optimized, compliant output for web/epub use.

(​PR #22198 includes a commit that could add EPUBCheck to our regular documentation testing, pending resolution of this issue. The NAV-011 warnings are probably ignorable.)

% cd docs
% make epub
% epubcheck _build/epub/Django.epub 
Validating using EPUB version 3.4 rules.
ERROR(RSC-005): _build/epub/Django.epub/_images/contribution_process.svg(4,36744): Error while parsing file: element "font" not allowed here; expected the element end-tag, text, element "ns1:switch", "ns1:trigger", "ns2:math", "ns3:a", "ns3:abbr", "ns3:address", "ns3:area", "ns3:article", "ns3:aside", "ns3:audio", "ns3:b", "ns3:bdi", "ns3:bdo", "ns3:blockquote", "ns3:br", "ns3:button", "ns3:canvas", "ns3:cite", "ns3:code", "ns3:data", "ns3:datalist", "ns3:del", "ns3:details", "ns3:dfn", "ns3:dialog", "ns3:div", "ns3:dl", "ns3:em", "ns3:embed", "ns3:fieldset", "ns3:figure", "ns3:footer", "ns3:form", "ns3:h1", "ns3:h2", "ns3:h3", "ns3:h4", "ns3:h5", "ns3:h6", "ns3:header", "ns3:hgroup", "ns3:hr", "ns3:i", "ns3:iframe", "ns3:img", "ns3:input", "ns3:ins", "ns3:kbd", "ns3:label", "ns3:link", "ns3:main", "ns3:map", "ns3:mark", "ns3:menu", "ns3:meta", "ns3:meter", "ns3:nav", "ns3:object", "ns3:ol", "ns3:output", "ns3:p", "ns3:picture", "ns3:pre", "ns3:progress", "ns3:q", "ns3:ruby", "ns3:s", "ns3:samp", "ns3:script", "ns3:search", "ns3:section", "ns3:select", "ns3:selectedcontent", "ns3:slot", "ns3:small", "ns3:span", "ns3:strong", "ns3:sub", "ns3:sup", "ns3:table", "ns3:template", "ns3:textarea", "ns3:time", "ns3:u", "ns3:ul", "ns3:var", "ns3:video", "ns3:wbr" or "svg" (with xmlns:ns1="http://www.idpf.org/2007/ops" xmlns:ns2="http://www.w3.org/1998/Math/MathML" xmlns:ns3="http://www.w3.org/1999/xhtml") or an element from another namespace
WARNING(NAV-011): _build/epub/Django.epub/nav.xhtml(462,81): "toc" nav must be in reading order; link target "topics/class-based-views/index.xhtml#basic-examples" is before the previous link’s target in spine order.
WARNING(NAV-011): _build/epub/Django.epub/nav.xhtml(922,64): "toc" nav must be in reading order; link target "howto/index.xhtml#templates-and-output" is before the previous link’s target in spine order.
WARNING(NAV-011): _build/epub/Django.epub/nav.xhtml(942,80): "toc" nav must be in reading order; link target "howto/index.xhtml#project-configuration-and-management" is before the previous link’s target in spine order.
WARNING(NAV-011): _build/epub/Django.epub/nav.xhtml(959,78): "toc" nav must be in reading order; link target "howto/index.xhtml#installing-deploying-and-upgrading" is before the previous link’s target in spine order.
WARNING(NAV-011): _build/epub/Django.epub/nav.xhtml(976,56): "toc" nav must be in reading order; link target "howto/index.xhtml#other-guides" is before the previous link’s target in spine order.
WARNING(NAV-011): _build/epub/Django.epub/nav.xhtml(1269,77): "toc" nav must be in reading order; link target "ref/class-based-views/index.xhtml#specification" is before the previous link’s target in spine order.
WARNING(NAV-011): _build/epub/Django.epub/nav.xhtml(1336,59): "toc" nav must be in reading order; link target "ref/contrib/index.xhtml#admin" is before the previous link’s target in spine order.

Check finished with errors
Messages: 0 fatals / 1 error / 7 warnings / 0 infos

If you don't want to install epubcheck through your package manager, it's also on PyPI:

% python -m pip install epubcheck
% python -m epubcheck _build/epub/Django.epub