doc: add some troubleshooting help

Inspired by #375.

* README: Add a section about troubleshooting installations.  Mention
GraphViz as optional dependency.
* doc/org/install.org: Mention the troubleshooting section from INSTALL.
13 jobs for next in 134 minutes and 10 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed alpine-gcc #29286

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passed arch-clang #29287

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passed arch-gcc-glibcxxdebug #29288

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passed debian-gcc-snapshot #29285

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passed debian-stable-gcc #29283

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passed debian-unstable-gcc-coverage #29284

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88.8%
passed debpkg-stable #29291

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passed debpkg-unstable #29292

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passed mingw-shared #29289

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passed mingw-static #29290

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passed rpm-pkg #29293

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  Publish
passed publish-rpm #29294
dput

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passed publish-unstable #29295
dput

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