docs(sse): add API reference page for sse.h (issue #271)

[Bhumika-1432006]

Fixes #271

Problem

include/cwist/net/http/sse.h exposes nine public functions, but there is
no reference page under docs/api/ and docs/API.md does not list SSE.

Fix

Add docs/api/sse.md in the same format as docs/api/query.md: a header
line, one section per function with its signature and a short
description, the cwist_sse_event_t type, the convenience macros, and a
short example for each of response mode and stream mode. Link it from
the module list in docs/API.md.

Verification

• make test_sse: "All SSE tests passed."
• Both code examples in the page compile with -Wall and were run. Output
  matches the page: the response-mode handler produces
  "id:1 / event:message / data:..." frames, and the stream-mode handler
  writes the 200 OK headers followed by "event:tick / data:1" and the
  ":heartbeat" comment over a socketpair.
• retry_ms of -2 returns err_i16 == -1, and retry_ms of 0 is emitted as
  "retry:0".
• ASCII check on the new file: clean.

Scope

• No changes to sse.h, sse.c or any test.
• The page describes the code as it is today. It does not describe
  planned work.

[gg582]

nice catch!