Document token permissions required by campaigns

Created by: mrnugget

This fixes #12888 by:

• linking to the code host specific documentation from the campaigns docs
• updating the GitLab documentation to make clear which permissions are required by campaigns
• adding a section to the Bitbucket Server docs to mention which permissions are required in general and by campaigns (some of that is duplicated from the schema, but I think it's much more discoverable up there)
• extending the comment in the Bitbucket Server schema to mention Write access for campaigns

Created by: codecov[bot]

Codecov Report

Merging #13948 into main will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff            @@
##             main   #13948    +/-   ##
========================================
  Coverage   51.09%   51.09%            
========================================
  Files        1515     1515            
  Lines       77246    77246            
  Branches     6895     7055   +160     
========================================
+ Hits        39467    39468     +1     
+ Misses      34216    34214     -2     
- Partials     3563     3564     +1     

Flag	Coverage Δ
#go	51.32% <ø> (-0.01%)
#integration	29.19% <ø> (+0.05%)
#storybook	18.24% <ø> (ø)
#typescript	50.53% <ø> (+0.01%)
#unit	33.90% <ø> (ø)

Impacted Files	Coverage Δ
.../internal/codeintel/resolvers/graphql/locations.go	83.50% <0.00%> (-2.07%)
...codeintel/bundles/persistence/cache/store_cache.go	92.30% <0.00%> (-1.29%)
web/src/tree/TreeRoot.tsx	87.93% <0.00%> (+6.89%)