Docs(Clients): Document transaction abort reason categories

[rahst12]

Description

Documents the new transaction abort reason categories that Dgraph reports to clients,
across the shared concept page and each client page.

When a commit aborts, the server now encodes why as a "<code>: <detail>" prefix on the
gRPC ABORTED status (and the HTTP error message), with <code> being one of conflict,
stale-startts, or predicate-move. This PR explains that format and the per-language
API:

• clients/index.md — new "Abort reasons" subsection: the three categories, the
  message format, and graceful degradation to UNKNOWN against older servers.
• clients/java.md — "Handling aborted transactions" using
  TxnConflictException.getReason() / AbortReason.
• clients/python.md — parallel section for pydgraph.AbortedError.reason /
  pydgraph.AbortReason.
• clients/raw-http.md — updated abort response example plus a table of the codes.

All four pages cross-link via the shared #abort-reasons anchor. Content is additive and
descriptive only.

Related PRs

• Server: feat(txn): surface transaction abort reasons to clients dgraph#9747
• Java client: feat: surface transaction abort reason on TxnConflictException dgraph4j#290
• Python client: feat: surface transaction abort reason on AbortedError pydgraph#307

Unreleased feature

⚠️ This documents behavior that has not shipped yet. Please hold merge until the
corresponding server and client releases are out.