Update folder structure for specification

[huijing]

Closes #222 Closes #339

This PR renames the original specification.html file into specification-respec.html and replaces public links to the specification to the statically generated version instead. This also introduces a folder structure that allows the file to be accessed without the .html at the end of the URL. The previous version of the specification was also generated for record-keeping purposes. And the instructions for developing the specification locally have been updated.

🎩 Top-hatting instructions (i.e. human testing that things are not broken)

1. Pull down this branch
2. Navigate to http://localhost:3000/specification and check that the specification is rendering correctly and it is NOT the respec version
3. Navigate to http://localhost:3000/specification-respec.html and check that it is the ReSpec version.
4. Navigate to https://localhost:3000/specification/versions/CG-DRAFT-web-monetization-20210317.html and check that it is version 1.0 of the specification

(or you could probably use the deploy preview since we turned that on ¯\_(ツ)_/¯)

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