Add a general article on osu! forums. Add forum-specific stubs

[TicClick]

General

• The changes are tested against the contribution checklist
• depends on Add the Community category #6438, ready for review otherwise (just don't merge it before ^ please)

Slightly related

• part of Add Article Stubs (Community) #3150
• closes Create Forums Stub #6225 (this has more information/better structure)
• I hope @cl8n and @abraker95 pick Expand the Community/Forum/Off-Topic stub #2804 up eventually

[venix12]

random 4am spellcheck of mine

[Gamingzone123]

looks good, thanks for picking up where i was unable to continue

[cl8n]

skimmed for now, most of it looks good

[cl8n]

seems not very related to Forum. it's been a long time since any of these things were displayed or implemented as features of forums

[TicClick]

yes (I linked the wrong article) but also no (custom titles are still there: https://user-images.githubusercontent.com/4686101/144977675-d76fb1c3-19c6-4c6e-bc0b-0f7820cbdbe4.png)

[cl8n]

i dont see how that makes it any more related to forums than, for example, the country flag in same screenshot. it's user-related stuff that appears all over the site, so seems weird to explain it here as if it's specifically related to the forums

[TicClick]

while flags are self-explanatory, it would be interesting to know how many (and which) custom titles there are. I agree it isn't specific to the forums, but hey, neither are user groups, or BBCode, or moderation stuff, and the further you go on this road, the closer the article becomes to "well you can leave long-lasting messages, like these on other web sites, we also have several message categories, but they are all alike, end of story"

mostly resolved and not sure if i'll have time to re-check soon

[Walavouchey]

this should clean it up