feat(js): Update JS tracing instrumentation docs

[mydea]

This updates the custom instrumentation docs for JS (tracing):

1. Link to new API pages for more complete list of APIs
2. Updating the "Custom Instrumentation" section on "Set up tracing" to link to APIs, instrumentation, and span metrics
3. Fixing one or two examples

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 4, 2025 9:43am

2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
changelog	⬜️ Ignored (Inspect)			Apr 4, 2025 9:43am
develop-docs	⬜️ Ignored (Inspect)			Apr 4, 2025 9:43am

[codecov]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	11.13MB	-9 bytes (-0.0%) ⬇️
sentry-docs-client-array-push	9.47MB	-6 bytes (-0.0%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: sentry-docs-client-array-push

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
static/chunks/pages/_app-*.js	-3 bytes	868.7kB	-0.0%
static/chunks/8165-*.js	-3 bytes	410.1kB	-0.0%
static/EhzcU1Qq46twrkMVd_lep/_buildManifest.js (New)	578 bytes	578 bytes	100.0% 🚀
static/EhzcU1Qq46twrkMVd_lep/_ssgManifest.js (New)	77 bytes	77 bytes	100.0% 🚀
static/tOeYYeBpLdzU-*.js (Deleted)	-77 bytes	0 bytes	-100.0% 🗑️
static/tOeYYeBpLdzU-*.js (Deleted)	-578 bytes	0 bytes	-100.0% 🗑️

view changes for bundle: sentry-docs-server-cjs

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
1729.js	-3 bytes	1.75MB	-0.0%
../instrumentation.js	-3 bytes	1.08MB	-0.0%
9523.js	-3 bytes	1.05MB	-0.0%