Add guide on changing a document's location
## What does this MR do?
Add a documentation styleguide when changing a document's location.
## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/3349
See merge request !4624
Improve ui consistency
* Decline idea of using gray buttons actively in UI
* remove icons from some buttons that already has text
* standardizes margin between buttons
First step in https://gitlab.com/gitlab-org/gitlab-ce/issues/18324
See merge request !4533
Resolve "SHA parameter in accept merge request API"
Add a `sha` parameter to the MR merge API, which must match the source SHA for the branch to be merged.
Also add the same parameter to the UI:

@DouweM and I discussed adding some smart feature to that, like updating the source SHA on navigating to the diff tab, but for now it will just require a refresh 😃Closes#14139.
See merge request !4414
The `sha` parameter is optional, and when present, must match the
current HEAD SHA of the source branch. Otherwise, the API call fails
with a 409 Conflict and a message containing the current HEAD for the
source branch.
Also tidy up some doc wording.
Amend jira service api docs [ci skip]
## What does this MR do?
Amends the API documentation for jira service integration
## Are there points in the code the reviewer needs to double check?
No
## Why was this MR needed?
The current documentation omits the username and password parameters
## What are the relevant issue numbers?
## Screenshots (if relevant)
See merge request !4382
Include Wiki attributes in Wiki page events webhook
The hook data we are sending is not 100% correct (we send details of the project, but should also send details of the "ProjectWiki" attributes like URLs and repositories relative to the wiki itself).
This is a follow up to #17506Fixes#17507
See merge request !4138
Enable Wiki Page events from Webhook admin UI in a Project
* Added UI to enable page wiki events
* Documentation for this event in Webhooks page
Fixes#17506
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See merge request !4135