leases.md states the wrong vendor-namespace segment count

[nficano]

docs/guides/leases.md:29-30 states that custom capability names must follow x-vendor.<vendor>.<cap> with "at least three dot-separated segments after the x-vendor. prefix". The implementation in packages/core/src/extensions.ts:12 defines VENDOR_EXTENSION_PATTERN = /^x-vendor(?:\.[a-z0-9_-]+){2,}$/, which requires at least two dot-separated segments after the prefix (so x-vendor.foo.bar and the canonical OTel key x-vendor.opentelemetry.tracecontext both validate). A reader following the docs would believe their two-segment vendor key was non-compliant when it is in fact accepted by the runtime, or — worse — would refuse to publish a perfectly valid name like x-vendor.opentelemetry.tracecontext because the docs say it needs another segment.

Fix prompt: edit docs/guides/leases.md:29-30 to say "at least two dot-separated segments after x-vendor." and add x-vendor.opentelemetry.tracecontext to the inline examples as the canonical motivating case. Cross-check docs/guides/vendor-extensions.md (it already states the rule correctly per packages/core/src/extensions.ts) so the two guides agree, and link the leases guide to the vendor-extensions guide for the single source of truth on namespace rules.

[narutamaaurum]

Opened PR #89 to correct the x-vendor. segment-count wording in docs/guides/leases.md, link that section back to the vendor-extensions guide, and call out x-vendor.opentelemetry.tracecontext as the canonical motivating example. This brings the leases guide back into line with packages/core/src/extensions.ts and the existing vendor-extensions guide.