Clarify "No default avatars" rule

[Hans5958]

Some clarifications regarding the "No default avatars" rule. The main change is adding concrete examples.

• Add simple excerpt
• Make language clearer
• Add concrete examples

TODO for reviewers/maintainers

• Decide whether to host all the images (incl. Piapro?)
• Decide whether more examples are needed (or less)
• Decide whether the contents (incl. excerpt and description) are sufficient

Preview

[Tonamu]

I think the preview & examples look great!

Regarding default avatars on Piapro, it seems like the default avatars have been changed at some point in time.
Maybe the previous default avatars could also be added to the default avatars list?
See: https://piapro.jp/pages/official_collabo/piapro2016/images/icon_sample.jpg
Found on: https://piapro.jp/pages/official_collabo/piapro2016/index

I don't know for sure, but it seem likely that if a default avatar was set before the default avatars were updated, the old default avatar still remains as the avatar of the Piapro profile. There are quite a many Piapro profiles still using the old default avatars.
For example: https://vocadb.net/Ar/14722, https://vocadb.net/Ar/23628 and https://vocadb.net/Ar/60062

[Hans5958]

@Tonamu, thanks for the review.

I actually registered on Piapro for this PR, just to check which ones are available. I couldn't find those that you linked (I found on my research that someone actually asked if they can find them). In fact, this is what appears on my profile settings.

Maybe I could search for users that has the new ones and link it here, but that would need some time. That doesn't mean this PR is not mergable for this.

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EDIT: I tried to find it by going to the user list in reverse:

• MEIKO: https://piapro.jp/meiko108
• KAITO: https://piapro.jp/sin_luka (ignore the luka part)
• Miku: https://piapro.jp/buka_a
• Rin: https://piapro.jp/em_es
• Len: https://piapro.jp/jukensei_a
• Luka: https://piapro.jp/jewel_fish