Draft: Convert coding guidelines to markdown

Experimental migration of the C++ coding guidelines (by @ssnyder) from the original org-format [1] to markdown. Mostly done using pandoc and few extra fixes using sed (see full command below).

Comparison:

• NEW: https://atlas-software.docs.cern.ch/staging-cppguidelines/coding-guidelines/
• OLD: https://atlas-computing.web.cern.ch/atlas-computing/projects/qa/draft_guidelines.html

TODO (in case we decide to make this our primary source for the coding guidelines)

• Author attribution
• Change a few code blocks to verbatim text
• Cleanup footnotes
• Fix and add a few missing anchors (already missing in org-format)
• Fix spellchecker
• Use a better name/URL so we could distinguish this from some future Python coding guidelines

[1] https://gitlab.cern.ch/ssnyder/coding-rules/-/blob/master/rules.org

To reproduce

cat rules.org |
    # Convert example code blocks to proper C++ code blocks
    sed 's/BEGIN_EXAMPLE/BEGIN_SRC cpp/' | sed 's/END_EXAMPLE/END_SRC/' |
    # Convert to markdown
    pandoc --shift-heading-level-by=1 --from=org --to=markdown_strict+backtick_code_blocks+footnotes |
    # Fix double quotes
    sed 's/\\`\\`/"/g' | sed "s/''/\"/g" |
    # Create proper anchors
    sed 's/\\\[<span id="\(.*\)"><\/span>\\\]/[[\1]](#\1){: #\1}/' > rules.md