I took all the quests from the noticeboard in white orchard, but they don't add a journal entry. Not on sidequests, witcher quests etc.

Is this bugged or am I missing something?

Not everything on noticeboards gives you quests. They are just to give immersion. But you can find Witcher contracts and normal sidequests on noticeboards (When someone writes something like "My brother has gone missing, please can anyone help me?").
After the prologue you will get more of these.

Once you took the initial six pieces, it replenishes over time; there are no new quests given however.

Noticed this as well. I don't think those notice board papers do anything. The only ones that give you a quest in your journal are witcher contracts.

They mark your map with "?" points of interest in the surrounding area.

They mark your maps with shops etc... didn't the tutorial tell you?

Can you grind those board missions ?

I have missed (not my fault, ask CDPR's beta testing team) 2 quests: Dead Funds and Deserter Gold). I can still loot the locations, but clearly I do not get any XP.

Same for me, the first 3 missions in the board create a quest task, but the other 7 missions!, not create nothing. And i completed quests and quests i find in the world map, but not gain XP, im blocked in 218 xp of level 2, but not gain more, not killing monsters, not completing missions.