docs: add new "main" Doxygen badge

docs/Doxygen-main-badge.svg

+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="96" height="20" role="img" aria-label="Doxygen: main"><title>Doxygen: main</title><linearGradient id="s" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="r"><rect width="96" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#r)"><rect width="59" height="20" fill="#555"/><rect x="59" width="37" height="20" fill="#007ec6"/><rect width="96" height="20" fill="url(#s)"/></g><g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" text-rendering="geometricPrecision" font-size="110"><text aria-hidden="true" x="305" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="490">Doxygen</text><text x="305" y="140" transform="scale(.1)" fill="#fff" textLength="490">Doxygen</text><text aria-hidden="true" x="765" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="270">main</text><text x="765" y="140" transform="scale(.1)" fill="#fff" textLength="270">main</text></g></svg>
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docs/Doxygen-master-informational.svg

-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="108" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="a"><rect width="108" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#a)"><path fill="#555" d="M0 0h59v20H0z"/><path fill="#007ec6" d="M59 0h49v20H59z"/><path fill="url(#b)" d="M0 0h108v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="110"> <text x="305" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="490">Doxygen</text><text x="305" y="140" transform="scale(.1)" textLength="490">Doxygen</text><text x="825" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="390">master</text><text x="825" y="140" transform="scale(.1)" textLength="390">master</text></g> </svg>
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