Developer Guide v2: Bulk-insert Data

[ericharmeling]

Related to #9065

from planning doc

Replacing: https://www.cockroachlabs.com/docs/v20.2/performance-best-practices-overview#bulk-insert-best-practices
Multi-row inserts (Level 2 - page section)
Misc notes:
Requires code snippet.
https://www.cockroachlabs.com/docs/v20.2/insert.html#performance-best-practices
https://www.cockroachlabs.com/docs/v20.2/performance-best-practices-overview#use-multi-row-insert-statements-for-bulk-inserts-into-existing-tables
https://www.cockroachlabs.com/docs/v20.2/insert.html#insert-multiple-rows-into-an-existing-table
https://www.cockroachlabs.com/docs/v20.2/insert-data.html#bulk-insert
Batch inserts (Level 2 - page section)
Misc notes:
Requires code snippet.
#7524
https://www.cockroachlabs.com/docs/v20.2/delete.html#batch-deletes
#5647 (comment)