Docs: Replace dependency sphinx-issues with builtin sphinx.ext.extlinks

[hugovk]

I recently discovered (from zoneinfo) that Sphinx comes with a builtin sphinx.ext.extlinks extension.

With some minimal config, it essentially does the same as sphinx-issues, so we can remove a dependency and install less, and easily add more roles if desired:

# sphinx.ext.extlinks
# This config is a dictionary of external sites,
# mapping unique short aliases to a base URL and a prefix.
# https://www.sphinx-doc.org/en/master/usage/extensions/extlinks.html
_repo = "https://github.com/python-pillow/Pillow/"
extlinks = {
    "cve": ("https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-%s", "CVE-%s"),
    "cwe": ("https://cwe.mitre.org/data/definitions/%s.html", "CWE-%s"),
    "issue": (_repo + "issues/%s", "#%s"),
    "pr": (_repo + "pull/%s", "#%s"),
}

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Also add a make htmlview target, which I'm familiar with from the CPython repos: it builds the docs and opens index.html in the browser.

And add the make html target as an alias for make doc: it's already in the help but not implemented.