update outdated links related to UAHF and use explicit MarkDown syntax

Summary: Some links in doc/abc/*md point toward a github repository that no longer exists. They have been updated to point to the local md file. Additionaly, some MarkDown syntax has been added to make links work on the HTML version auto-generatedfor bitcoinabc.org

Test Plan:

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/D7486