Reformat template files for `rebar3 new lfe-main`

[joeljuca]

Hi!

First time contributing here :) So, I've tried rebar3 new lfe-main today to bootstrap a new LFE project, and I promptly noticed minor inconsistencies in the README.md Markdown. After looking around for a minute or so, I think I've found the right files to fine-touch it.

I've:

• reformatted the README.md.tpl using "best practices" (reads: Prettier)
• adjusted consistent heading levels for each section (Build, REPL, Tests, etc.)
• Git-ignored common env-specific configuration files (.env, and direnv's .envrc)
• reformatted the cicd.yml template using Prettier, and
  • removed step names (when absent, the command itself will be the "name" – which tells exactly what's being executed)
  • updated actions/checkout to use v4
  • changed schedule/cron to run the periodic job at 3 am each 1st day of the month (a more predictable datetime)
  • removed branch definitions for pushes and PRs (it seemed highly based on branches used by lfe/rebar3)
  • renamed job names to main and legacy (it seemed more appropriate)

These bite-sized changes might help generate an LFE project that feels more organized, but these are mostly recommendations. Let me know if something doesn't look good and I'll fix it.