Add join algorithms design, developer and cost model docs

[Dileepadari]

Fills in the JOIN Algorithms project section, which until now was a stub containing only a heading.

Three new pages under content/storage-engine/projects/join-algorithms/:

Page	Covers
Design Rationale	Why each significant decision was taken and what it prevents - the join key encoding, NULL handling, predicate splitting, spill scoping, memory accounting
Developer Guide	Module map, invariants, how to add an algorithm, how to run and read the tests
Cost Model	The formulas, the coefficients and how they were calibrated, and what remains an estimate

The existing join-algorithms.md becomes the landing page, with a short overview of the operators and join types plus an index of the three pages. Its previous content was a bare # Join Algorithms heading.

Related

• Joins pr in main repo - Rewrite the join subsystem against the current storage engine (replaces #21) RookDB#49