/claim #851

Contributes to #851.

This adds another focused docs-revamp slice for @cycle/http. It expands the HTTP API page with:

• driver setup with the DOM driver included in the example
• basic request/response flow using categories
• common request object shapes for GET, POST, headers, query, upload, and progress
• guidance on select(), response metastreams, the response$.request field, flatten(), and flattenConcurrently()
• examples for custom HTTPSource.filter() usage
• xstream and RxJS error-handling examples on inner response streams
• HTTP isolation semantics with a component example

I also mirrored the Markdown content into the checked-in docs/api/http.html wrapper because the docs generator is currently blocked in this checkout.

Verification run locally:

• node --check .scripts/make-api-docs.js
• git diff --check
• embedded-markdown consistency check between docs/content/api/http.md and docs/api/http.html

I also tried regenerating the rendered API HTML with:

• node .scripts/make-api-docs.js http

That fails in the existing docs toolchain because the installed GitHub tarball for dox throws TypeError: parse is not a function, matching the same pre-existing generator failure seen on the other docs-revamp branches.