docs: document the Python 3.11-only requirement

## Problem

The installation section presents `pip install hi-pdf-parser` without stating that the package currently supports Python 3.11 only. The published metadata and `pyproject.toml` require `>=3.11,<3.12`.

This affects the current `main` commit `c6478aae9ab50a13fc54c0599a0a459396c15b99` and release `0.0.5`.

## Reproduction

```console
$ python3.12 -m venv .venv
$ .venv/bin/python -m pip install --no-deps hi-pdf-parser==0.0.5
ERROR: Ignored the following versions that require a different python version: 0.0.4 Requires-Python >=3.11,<3.12; 0.0.5 Requires-Python >=3.11,<3.12
ERROR: Could not find a version that satisfies the requirement hi-pdf-parser==0.0.5
ERROR: No matching distribution found for hi-pdf-parser==0.0.5
```

The same incompatibility applies to Python 3.13 and newer while the current upper bound remains in place.

## Expected documentation

The installation section should state the supported Python version before the generic pip command, so users can select a compatible interpreter without first encountering a resolver failure.

## Suggested change

Add a concise Python 3.11 requirement to the README installation section, keeping it aligned with `project.requires-python` and `.python-version`.

Start with the installation section in README and compare its stated requirement with project.requires-python in pyproject.toml and .python-version. Add a concise Python 3.11-only requirement before the pip command, keeping it aligned with those project settings. Done means users can identify the compatible interpreter before installing.

Written by the indexing model from the issue text.