Add a progress bar with a maximum for the rendering phase

[Amoifr]

Fixes #19, and removes the // TODO: create a proper progress bar for rendering in BuildProgressListener.

Today the parsing phase gets a real progress bar, while rendering only prints a static note:

 ! [NOTE] Rendering the HTML files...

Now it gets a bar that knows where it's going:

 ! [NOTE] Rendering 3 HTML files

 3/3 [============================] 100%

How, since the parser has no per-document render event. Documents::render() loops over the documents without dispatching anything, and DocumentNode::renderDocument() bypasses Node::render(), so there's no "document rendered" signal to hook into. But there is enough to work with:

• the maximum comes from PreBuildRenderEvent → getBuilder()->getDocuments()->getAll(), which is exactly the set of documents about to be rendered;
• the progress is tracked on POST_NODE_RENDER by looking at the file each rendered node belongs to, advancing once per file — the same "files already seen" approach the parsing bar already uses in postParseDocument();
• the bar is finished on POST_BUILD_RENDER.

Nodes are also rendered outside the rendering phase, so the listener ignores them until the bar exists.

Tests: BuildDocsCommandTest now asserts both bars announce their file count and reach it. Verified the assertions fail without the change (the output only contains the old static note). Full suite green: 68 tests, 157 assertions.

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Unrelated, but I ran into it while testing this and it seems worth flagging: bin/docs-builder is currently broken on Symfony 8 — Application::add() was removed in favour of addCommand(), so the binary dies immediately:

PHP Fatal error: Call to undefined method Symfony\Component\Console\Application::add() in src/Application.php:38

The test suite doesn't catch it because it drives the command through CommandTester rather than the binary. Happy to send a separate PR if you'd like.