v0.29: Updates to the `filter` search parameter

[guimachiavelli]

v0.29 will expand functionality of filter:

• New IN operator;
• New EXIST operator;
• Updated NOT operator behavior;
• Filter values identical to filter operators must be enclosed in quotation marks (x = 1 AND y = 0 vs x = "AND").

Tasks

• update /reference/api/search.md
• update /learn/advanced/filtering_and_faceted_search.md
• update learn/getting_started/filtering_and_sorting.md
• search for other articles mentioning and update them if necessary

References

Issue: meilisearch/meilisearch#2580
SMEs: @loiclec, @curquiza