Add --root option to fsdocs watch for remote/proxy hosting

Closes #924

Previously fsdocs watch always forced root = http://localhost:<port>/,
ignoring any user-supplied root parameter. This makes the generated pages
unusable when served via GitHub Codespaces port-forwarding, a reverse proxy,
or any other non-localhost environment.

This PR adds:
- A --root option to WatchCommand exposing a user-specified root URL override
- An abstract root_override_option on CoreBuildOptions (default None)
- Modified watch-mode root logic: uses the --root value when provided,
  otherwise falls back to http://localhost:<port>/ as before

Usage:
  fsdocs watch --root /                       # root-relative URLs
  fsdocs watch --root https://example.com/docs/   # absolute remote URL

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -3,6 +3,7 @@
 ## [Unreleased]
 
 ### Added
+* Add `--root` option to `fsdocs watch` to override the root URL for generated pages. Useful for serving docs via GitHub Codespaces, reverse proxies, or other remote hosting where `localhost` URLs are inaccessible. E.g. `fsdocs watch --root /` or `fsdocs watch --root https://example.com/docs/`. When not set, defaults to `http://localhost:<port>/` as before. [#924](https://github.com/fsprojects/FSharp.Formatting/issues/924)
 * Add `dotnet fsdocs convert` command to convert a single `.md`, `.fsx`, or `.ipynb` file to HTML (or another output format) without building a full documentation site. [#811](https://github.com/fsprojects/FSharp.Formatting/issues/811)
 * `fsdocs convert` now accepts the input file as a positional argument (e.g. `fsdocs convert notebook.ipynb -o notebook.html`). [#1019](https://github.com/fsprojects/FSharp.Formatting/pull/1019)
 * `fsdocs convert` infers the output format from the output file extension when `--outputformat` is not specified (e.g. `-o out.md` implies `--outputformat markdown`). [#1019](https://github.com/fsprojects/FSharp.Formatting/pull/1019)

src/fsdocs-tool/BuildCommand.fs

@@ -1560,9 +1560,15 @@ type CoreBuildOptions(watch) =
         // Adjust the user substitutions for 'watch' mode root
         let userRoot, userParameters =
             if watch then
-                let userRoot = sprintf "http://localhost:%d/" this.port_option
-
-                if userParametersDict.ContainsKey(ParamKeys.root) then
+                let userRoot =
+                    match this.root_override_option with
+                    | Some r -> r
+                    | None -> sprintf "http://localhost:%d/" this.port_option
+
+                if
+                    userParametersDict.ContainsKey(ParamKeys.root)
+                    && this.root_override_option.IsNone
+                then
                     printfn "ignoring user-specified root since in watch mode, root = %s" userRoot
 
                 let userParameters =
@@ -2325,6 +2331,9 @@ type CoreBuildOptions(watch) =
     abstract port_option: int
     default x.port_option = 0
 
+    abstract root_override_option: string option
+    default x.root_override_option = None
+
 [<Verb("convert",
        HelpText =
            "convert a single document (.md, .fsx, .ipynb) to HTML or another output format without building a full documentation site")>]
@@ -2504,3 +2513,12 @@ type WatchCommand() =
 
     [<Option("port", Required = false, Default = 8901, HelpText = "Port to serve content for http://localhost serving.")>]
     member val port = 8901 with get, set
+
+    override x.root_override_option = if x.root = "" then None else Some x.root
+
+    [<Option("root",
+             Required = false,
+             Default = "",
+             HelpText =
+                 "Override the root URL for generated pages. Useful for reverse proxies or GitHub Codespaces. E.g. --root / or --root https://example.com/docs/. When not set, defaults to http://localhost:<port>/.")>]
+    member val root = "" with get, set