Broken references on the BC page

[alexislefebvre]

Here: https://symfony.com/doc/4.4/contributing/code/bc.html#changing-interfaces

It it shown “Yes [2]_” instead of a link to the reference.

It affects all the maintained versions.

[javiereguiluz]

The root problem is that our RST parser doesn't support footnotes. We considered that this is OK because the parser result is not broken (you can still see the footnote numbers).

However, there are other issues ... such as some footnotes mysteriously disappearing from the generated HTML content. E.g. where are footnotes 6 and 9?

So, in #17144 I propose to remove footnotes and use normal internal references instead.