docs: add EBNF grammar for the reader syntax (doc/grammar.ebnf)
Specify the surface syntax Jolt's reader accepts as an EBNF grammar — the
syntactic half of the contract (behavioural half is test/spec/). Grounded in
src/jolt/reader.janet and verified against the running reader: whitespace
(comma included), comments, #_ discard, scalars (nil/bool/number incl 0x hex &
floats/string/char incl \uNNNN \oNNN & named/keyword incl ::auto/symbol),
collections, reader macros (quote/syntax-quote/unquote/~@/deref/metadata), and
dispatch (#{}, #(), #', #"regex", #?/#?@, tagged #inst/#uuid). Jolt-vs-Clojure
deviations noted inline (no ratios/radix/BigInt literals; PEG regex limits).
Referenced from README.
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