more fixes in Markdown files

[michelou]

Follow up of PR #10767, #10826, #10860, #10875, #10953, #11016 and #11062.

Fixes more issues found in Markdown files in docs/reference/, e.g.

• corrected typos (eg. string -> String)
• add a few external links (e.g. Scala 3 API)
• improved example in section "Type Test" (§6.17.3 in generated PDF file)

PS. The above changes can also be viewed in the PDF document scala3_reference.pdf (955 Kb) generated with Pandoc 2.11 in ~1:10 minute.

[liufengyun]

LGTM, thanks @michelou 👍