Import/export docs

[prrao87]

This PR adds detailed docs for importing and exporting data from/to Kùzu. The usage of COPY FROM and COPY TO statements and others are described in detail.

[ray6080]

LGTM!

One additional thing we can document on COPY (not sure if we have it somewhere already or not, can you double check?): we support copy partial columns from a file as done in this PR.
An example is this

CREATE REL TABLE knows1 (FROM person TO person, nullDouble DOUBLE, date DATE, meetTime TIMESTAMP,
     validInterval INTERVAL, comments STRING[], summary STRUCT(locations STRING[],
    transfer STRUCT(day DATE, amount INT64[])), notes UNION(firstmet DATE, type INT16, comment STRING));
COPY knows1 (date, meetTime, validInterval, comments, summary, notes) FROM "dataset/tinysnb/eKnows.csv" ;

Thanks!

[prrao87]

Will address the partial column update in another PR.