docs(config): document 4 missing sections in example config file

[SteveLauC]

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md

---

What does this PR do

1. Document 4 missing sections in config.example.toml
   1. post_command
   2. yarn
   3. vim
   4. vagrant
2. Reorder the sections in config.example.toml so that they correspond to the order used by struct ConfigFile

[codecov]

Codecov Report

Merging #485 (be3ba06) into master (9cd155f) will decrease coverage by 0.07%.
The diff coverage is 2.27%.

@@            Coverage Diff            @@
##           master    #485      +/-   ##
=========================================
- Coverage    3.50%   3.43%   -0.07%     
=========================================
  Files          27      31       +4     
  Lines        3141    4194    +1053     
=========================================
+ Hits          110     144      +34     
- Misses       3031    4050    +1019     

Impacted Files	Coverage Δ
src/command.rs	0.00% <0.00%> (ø)
src/ctrlc/interrupted.rs	0.00% <ø> (ø)
src/error.rs	0.00% <0.00%> (ø)
src/execution_context.rs	0.00% <0.00%> (ø)
src/executor.rs	0.00% <0.00%> (ø)
src/main.rs	0.36% <0.00%> (-0.06%)	⬇️
src/report.rs	0.00% <0.00%> (ø)
src/runner.rs	0.00% <0.00%> (ø)
src/steps/containers.rs	0.00% <0.00%> (ø)
src/steps/emacs.rs	0.00% <0.00%> (ø)
... and 20 more