PEP 558: fix footnote references

[hugovk]

Fix these footnotes:

pep-0558.rst:1299: WARNING: Footnote [2] is not referenced.
pep-0558.rst:1323: WARNING: Footnote [10] is not referenced.
pep-0558.rst:1326: WARNING: Footnote [11] is not referenced.
pep-0558.rst:1329: WARNING: Footnote [12] is not referenced.
pep-0558.rst:1332: WARNING: Footnote [13] is not referenced.

[2] was removed in f057675

The others were written like [8,13]_ and [10,11,12]_ instead of [8]_ [13]_ and [10]_ [11]_ [12]_

Also:

• Remove redundant emacs metadata
• Fix code formatting (e.g. .. code-block:: c)
• Fix a couple of typos
• Update BPO links to GH
• Convert the references to regular links

Preview

https://pep-previews--2754.org.readthedocs.build/pep-0558/

[CAM-Gerlach]

Checked the preview and changes and they LGTM, thanks @hugovk

[hugovk]

Thanks for the reviews!