Detect line endings

[asgrim]

At the moment, the tool crashes if it encounters a file with CRLF (\r\n) line endings. The error is misleading (it says it can't find the title of the page), but we should detect these line endings being used and reject it with a more helpful error message.

Expected behaviour

• An error message saying that CRLF (\r\n) are not supported. Suggested remediation can be to use dos2unix <filename>.

Actual behaviour

[2021-10-06T16:23:13.821568+00:00] cli.INFO: Writing HTML output to /docs-package/index.html [] []
PHP Fatal error:  Uncaught RuntimeException: First line of markdown file <filename> did not start with "# "... in /app/src/DocbookPage.php:71
Stack trace:
#0 /app/src/DocbookPage.php(83): Roave\DocbookTool\DocbookPage->determineTitleFromContent()
#1 /app/vendor/twig/twig/src/Extension/CoreExtension.php(1541): Roave\DocbookTool\DocbookPage->title()
#2 /app/vendor/twig/twig/src/Environment.php(358) : eval()'d code(226): twig_get_attribute()
#3 /app/vendor/twig/twig/src/Template.php(394): __TwigTemplate_2904b74a3842d9b80ea20a5768e5df794dec35bfd57ea5620521eb0ac55c9c23->doDisplay()
#4 /app/vendor/twig/twig/src/Template.php(367): Twig\Template->displayWithErrorHandling()
#5 /app/vendor/twig/twig/src/Template.php(379): Twig\Template->display()
#6 /app/vendor/twig/twig/src/TemplateWrapper.php(40): Twig\Template->render()
#7 /app/vendor/twig/twig/src/Environment.php(277): Twig\TemplateWrapper->render()
#8 /app/src/Writer/SingleStaticHtmlWriter.php(34): Twig\Environment->render()
#9 /app/src/WriteAllTheOutputs.php(22): Roave\DocbookTool\Writer\SingleStaticHtmlWriter->__invoke()
#10 /app/bin/docbook-tool.php(55): Roave\DocbookTool\WriteAllTheOutputs->__invoke()
#11 /app/bin/docbook-tool.php(59): Roave\DocbookTool\{closure}()
#12 /app/bin/docbook-tool(6): require_once('...')
#13 {main}

Next Twig\Error\RuntimeError: An exception has been thrown during the rendering of a template ("First line of markdown file <filename> did not start with "# "..."). in /docs-src/templates/online.twig:179
Stack trace:
#0 /app/vendor/twig/twig/src/Template.php(367): Twig\Template->displayWithErrorHandling()
#1 /app/vendor/twig/twig/src/Template.php(379): Twig\Template->display()
#2 /app/vendor/twig/twig/src/TemplateWrapper.php(40): Twig\Template->render()
#3 /app/vendor/twig/twig/src/Environment.php(277): Twig\TemplateWrapper->render()
#4 /app/src/Writer/SingleStaticHtmlWriter.php(34): Twig\Environment->render()
#5 /app/src/WriteAllTheOutputs.php(22): Roave\DocbookTool\Writer\SingleStaticHtmlWriter->__invoke()
#6 /app/bin/docbook-tool.php(55): Roave\DocbookTool\WriteAllTheOutputs->__invoke()
#7 /app/bin/docbook-tool.php(59): Roave\DocbookTool\{closure}()
#8 /app/bin/docbook-tool(6): require_once('...')
#9 {main}
  thrown in /docs-src/templates/online.twig on line 179
make: *** [Makefile:64: build-docs] Error 255