docs: Update documentation for retrieving a list of all templates

[renshuki]

Fixes #289

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Update USAGE.md for retrieving a list of all templates (dynamic & legacy)
• Update /example file for retrieving a list of all templates (dynamic & legacy)

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #295 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #295   +/-   ##
=======================================
  Coverage   70.95%   70.95%           
=======================================
  Files           3        3           
  Lines         451      451           
=======================================
  Hits          320      320           
  Misses        127      127           
  Partials        4        4

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[ackleymi]

Can this get merged? I lost time on this.

[renshuki]

@thinkingserious do you think this could be merged?

[thinkingserious]

Hello @renshuki,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX