[Repo Assist] docs: add HTTP Authentication section to Http.fsx

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Adds a new HTTP Authentication section to docs/library/Http.fsx, covering the three most common authentication patterns using the existing HttpRequestHeaders helpers and customizeHttpRequest.

What's documented

Basic authentication

Http.RequestString(
    "(api.example.com/redacted),
    headers = [ HttpRequestHeaders.BasicAuth "myUsername" "myPassword" ]
)

Bearer / token authentication (OAuth 2.0, PATs)

Http.RequestString(
    "https://api.github.com/user",
    headers =
        [ HttpRequestHeaders.Authorization(sprintf "Bearer %s" token)
          HttpRequestHeaders.UserAgent "MyApp" ]
)

Windows / NTLM integrated authentication

Http.RequestString(
    "(intranet.example.com/redacted),
    customizeHttpRequest = fun req ->
        req.UseDefaultCredentials <- true
        req
)

Why

Issue #158 has been open since 2013 asking for HTTP authentication support. The required functionality (HttpRequestHeaders.BasicAuth, HttpRequestHeaders.Authorization, and customizeHttpRequest) was already implemented — it just wasn't documented. This PR adds the missing documentation.

No code changes are needed; this is a documentation-only PR.

Closes #158

Test Status

Documentation-only change; no build or test failures expected.

Generated by Repo Assist for issue #158

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