MNT: Ignore generated fast_converters.c - #19686

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No need to wait for doc. (I think we spamming RTD too much lately with the feature freeze frenzy so now it a bit wonky.)

I am also going to backport this to v7.2.x for the following scenario:

• have build files from later branches with this C file
• switch to v7.2.x for some hot fix
• git now say this file is untracked

@meeseeksdev backport to v7.2.x