nixos/docs: cache mergeJSON md conversion on baseOptionsJSON
nixos/docs: cache mergeJSON md conversion on baseOptionsJSON

with ever more options being markdown rather than docbook the conversion
time is starting to become a significant factor of doc build time.
luckily we can pre-convert all nixos option docs to MD and cache the
result of this conversion, then merge the already-converted json file
with user option docs. we leave options.json unconverted to keep it as
close to the actual nix code as possible.

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