fix MarkDown links and code formatting

Summary:
Continuation of D7475. Use explicit markdown syntax for links to improve make them render correctly both in github and auto-generated html pages on bitcoinabc.org.

I also changed a URL in doc/README.srt to point the ABC developer doxygen documentation rather than the Core one.

Test Plan:
Generate the abc_md_docs files:

SRC_DIR="/home/pierre/git/bitcoin-abc"
TOPLEVEL="/home/pierre/git/bitcoin-abc-website"
pushd "${SRC_DIR}"
ABC_MD_DOCS_BASE="${TOPLEVEL}/abc_md_docs"
mkdir -p "${ABC_MD_DOCS_BASE}"
FILES=($(git ls-files "*.md"))
for FILE in "${FILES[@]}"
do
  FILE_DST="${ABC_MD_DOCS_BASE}/${FILE}"
  mkdir -p "$(dirname ${FILE_DST})"

  NAME="${FILE}" \
  PERMALINK="/${FILE%.*}.html" \
  envsubst < "${TOPLEVEL}/scripts/md_docs_frontmatter.yml.in" > "${FILE_DST}"
  cat "${FILE}" >> "${FILE_DST}"
done
popd

Then test the links by running:

bundle exec jekyll serve

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7485