chore(docs): add section for Custom Type Declarations #17847

Job	Run time
Determine changes	11s
validate-docs-links	13s
build-native / build	20s
build-next / build	27s
test unit	0s
types and precompiled	0s
test cargo integration	0s
test cargo unit	0s
rust check	0s
test turbopack dev	0s
test turbopack integration	0s
test next-swc wasm	0s
test dev	0s
test firefox and safari	0s
test integration	0s
test ppr integration	0s
test prod	0s
test ppr dev	0s
test ppr prod	0s
lint / build	1m 9s
test cargo benchmarks / build	31s
report test results to datadog	0s
thank you, next	1s
	2m 52s

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