Alias the command markdown-cli as markdown2tex

[Witiko]

As discussed with @kberry on the TeX Live mailing list, a PR that closes this ticket should make the following changes:

• Update the documentation to mention markdown2tex as the preferred name of the markdown CLI executable, following up on commit b630714 from PR Make markdown-cli executable and symlink it to system directories #537.
• Add a new file markdown2tex.lua, which aliases markdown-cli.lua and include it in files Makefile and Dockerfile.
• Update section "Notes to Distributors" in README.md to request that the file markdown2tex.lua be made executable.