[AISP-1030] Document beforeTrack plugin callback in mobile tracker docs

[snowplow-claude-review]

Implementation task — AISP-1030

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What

Add a new "Intercepting events before they are tracked" section to the mobile tracker plugin documentation, covering the beforeTrack callback now available in both the iOS and Android trackers.

Why

beforeTrack is a new plugin callback that lets host apps overwrite or add event payload fields before events are sent to the collector — for example, stripping sensitive fields when a user has only partially consented. Without documentation the feature is not discoverable, and three non-obvious constraints (thread-safety, overwrite/add-only semantics, raw key names) need to be surfaced prominently to prevent misuse.

Approach

Insert a new subsection immediately before the existing "Inspecting events after they are tracked" (afterTrack) section in the mobile plugin docs, mirroring its tab-based code-example structure (iOS Swift / Android Kotlin / Android Java). The primary code example should illustrate consent-based field stripping. Three constraints must be called out explicitly as noted in Changes below.

Depends on

• Both the snowplow-ios-tracker and snowplow-android-tracker beforeTrack PRs must be merged and released before this PR is merged, so that code examples reference the released API rather than unreleased snapshots.

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Changes

• docs/sources/mobile-trackers/plugins/index.md — MODIFY — Add a new ### Intercepting events before they are tracked section immediately before the existing ### Inspecting events after they are tracked section (currently around line 162). The section must include:
  1. A consent-based field-stripping code example in iOS (Swift) and Android (Kotlin + Java) tabs, using the same tab structure as the afterTrack section.
  2. An explicit callout that on iOS the closure runs on the tracker's InternalQueue: a blocking call inside the closure (e.g. synchronous I/O or a lock wait) will stall event tracking.
  3. A callout that v1 supports overwrite and add only — returning a key with a new value overwrites it in the payload; key removal is not supported in this release.
  4. A note that the payload is keyed by raw tracker field names (e.g. url, se_ca) rather than the final collector field names.