Update source/core/indexes.txt

Added index caveats wrt $or
initial cut of mongoperf doc
DOCS-987 break up shard page into concepts, tutorials, refs
DOCS-987 break out sharding-requirements into own page
DOCS-987 change link format on shard page, create shard command ref
DOCS-987 many changes
DOCS-987 ongoing
DOCS-987 ongoing
DOCS-987 semi-solution for administration/sharding doc
DOCS-987 small
DOCS-987 ongoing edits
DOCS-987 added topic: view cluster configuration
DOCS-987 merged duplicate add shard content
DOCS-987 minor
DOCS-987 merged duplicate remove shard contents
DOCS-987 typo
DOCS-987 added explanatory text to main sharding page
DOCS-987: edits and revision to sharding reorganization
minor: DOCS-1018 corretion to formating glitch
build: minor tweak to get better link building