Broken links, typos, and improvements

[glmvc]

I like the introduction package very much, it shows many aspects and concepts of TYPO3.
But some links redirect to 404 and 403 pages and there are some small typos and missing informations.
Here are my findings, maybe someone can check this for the next "rollout" :)

• Root page (Home)
  • Button links under "Possibilities" and "Examples"
  • "Professional Service Listing" link in the footer section under "Professional Services"
• "Features" page
  • "Read TYPO3 Install Guide" button link under "Runs everywhere"
• "Customize" page
  • "your TYPO3 service provider" text link under "Add functionality to the core system"
  • "Fluid" button link under "Extensive Configuration Options"
• "Pages" page
  • "ste by step tutorial" typo under "Ready to create your own Template?"
• "Rich Text" page
  • Link examples (except external, mail etc.) should maybe point to the page itself, like it's the case on other content element pages?
  • Mention phone links?
  • Explain what "folder" links are for (nginx server just shows a 404)?
• "Headers" page
  • Add Header 6 levels?
• "Bullet List" page
  • "Definition List" not mentioned in lead text
• "Audio" page
  • "A short video explaining what..." under "What is TYPO3?" - use "audio" instead?
• "Media/Images" page
  • "back-end" without a hyphen as used by TYPO3's wording?
• "Media/Media" page
  • "back-end" without a hyphen as used by TYPO3's wording?
  • "control over image formats, sizes..." - use "media"?
• "Accordion" page
  • "Accordions" heading - use singular like it's the case on the other interactive element pages?
• "Search" page
  • The Search plugin shows "Oops, an error occurred!"
• "Insert records" page
  • "With the "Record insert" element..." - the element is called "Insert records"
  • "...existing content elements | different places..." - the word "from" seems to miss here
• "HTML" page
  • "The following contains address information..." - no address element following
  • Numbered list - "This is the last item..." sentence should be placed to number 10
  • The <tt> element is a deprecated element and shouldn't be used anymore (under "Text-level markup" 2 times)
  • "the command cat filename..." - the first "filename" word should be within the code element?
  • both "YUI" links seem broken
  • I like the <marquee>, but it's also a deprecated element 😭
• "Frames" page
  • Can't see rulers and indents
• "Images with links" page
  • "back-end" without a hyphen as used by TYPO3's wording?
• "404" page
  • "search" link under "What to do" is incorrect

I've installed the package as described in the documentation article "Introduction Package" on a Composer-based installation using DDEV ("typo3/cms-base-distribution:^13").

I know these are very specific points and mostly small matters, but I hope they help nonetheless.
Thanks for the nice work!