Split, shift, recombine basic MR information

What does this MR do?

In the quest to continue rebuilding the main merge request page (https://docs.gitlab.com/ee/user/project/merge_requests/) into something that's actually useful, instead of depending on it + https://docs.gitlab.com/ee/user/project/merge_requests/getting_started.html in tandem, continue to shift information around to new homes.

• Adds doc/user/project/merge_requests/commits.md as a new page as a companion to doc/user/project/merge_requests/changes.md.
• Adds doc/user/project/merge_requests/widgets.md as a new page as a home for all the various things we need to say about the merge request widgets area. Small now. Won't stay that way.
• Builds a new subheading explaining the secondary nav (overview, commits, pipelines, changes) on a merge request
• Adds a new subheading explaining the existence of the sidebar on the Overview page.

The new pages aren't great / polished / complete, but the amount of random crap dumped in getting_started.md, reviews/index.md, and changes/index.md is starting to drop, and index.md might even become useful someday at this rate...

Related to #324953 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #324953 (closed)