Fix broken API links, fixes #4463

minor contributing guide updates

* issues & pr in English
* appropriate language
* typo & format fix
* not catch all pull request
* update changelog

Added client list to API doc

Updating documentation to reflect #3836 (resolving #4443)

Add 'services' to path blacklist

Fix misspelling in social accounts tip

move 'make config/database.yml readable to git only' instruction

I've moved the instruction in question to the the 
'Configure GitLab DB settings'. Can't change permission to a file that doesn't already exist 

Editable notes

Couple of UI additions

fixed nav bar for empty repos (fixes #4375)

Adding detailed documentation of the markdown syntax.

Closes #4331