Generated documentation contains some HTML errors

### Prerequisite Checklist

- [X] I searched for [existing issues](https://github.com/search?q=repo%3ASFML%2FSFML&type=issues) to prevent duplicates
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- [X] I am here to report an issue and not to just ask a question or look for help (use the [forum](https://en.sfml-dev.org/forums/index.php#c3) or [Discord](https://discord.gg/nr4X7Fh) instead)

### Describe your issue here

I noticed a few issues with the doxygen headers / footers. Most can be seen by putting the documentation index page into the W3C validator. You can also see them with the live 2.6.1 documentation: https://validator.w3.org/check?uri=https%3A%2F%2Fwww.sfml-dev.org%2Fdocumentation%2F2.6.1%2Findex.php&charset=%28detect+automatically%29&doctype=Inline&group=0



---



header.html.in:

```html

        <meta http-equiv="Content-Type" content="text/html;"/>

        <meta charset="utf-8"/>

```



Three problems here:

* `meta charset` is a HTML5 thing. It's not valid in XHTML.

* Using both of these elements together is illegal in HTML5 in any case.

* If you don't specify a `charset` in your `Content-Type` then the default is ASCII and not UTF-8 like you want.



---



footer.html:

```html

        </div>

```



There's an unmatched `</div>` on line 1 of the footer.



---



footer.html:

```html

                SFML is licensed under the terms and conditions of the <a href="https://www.sfml-dev.org/license.php">zlib/png license</a>.<br>

```



The `<br>` is not valid XHTML.

### Your Environment

- OS / distro / window manager: Debian unstable

- SFML version: master (but also affects 2.6.1)

- Compiler / toolchain: GCC 13

- Special compiler / CMake flags: `-DSFML_BUILD_DOC=1`

### Steps to reproduce

1. Build SFML with documentation

2. Run the index page through https://validator.w3.org/

### Expected behavior

No errors

### Actual behavior

Generates errors

[]