DOP-4669: add env var for enabling dark mode

[seungpark]

Stories/Links:

DOP-4669

This PR adds logic to fetch AWS SM Param Store variables and use it to populate environment variables in the autobuilder + snooty front end

Notes

• These key/values have been added to the param store manually in S3, with dotcomprd and prd values being false as described in ticket

README updates

• • This PR introduces changes that should be reflected in the README, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README

[mayaraman19]

just to clarify - when GATSBY_ENABLE_DARK_MODE is true, does this mean dark mode is on or that the toggle is visible?

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[seungpark]

just to clarify - when GATSBY_ENABLE_DARK_MODE is true, does this mean dark mode is on or that the toggle is visible?

this means that the ability to turn on dark mode should be enabled on the front end :D this does not turn on dark mode automatically

[branberry]

Hey Seung! Just one small comment

[branberry]

I changed my mind, I'll approve it because my comment is really minor. Everything else looks good!