[ci] [docs] add Oliver to CODEOWNERS

[jameslamb]

Adds LightGBM's newest maintainer, @borchero , to CODEOWNERs.

Removes some unnecessary and out-of-date information from https://lightgbm.readthedocs.io/en/latest/FAQ.html.

[jameslamb]

@borchero this will automatically add you as a reviewer on every PR.

To be sure we set the right expectation...you do not need to review every PR! We require 1 approval from a maintainer to merge. In some situations, like more controversial changes or changes where particular maintainers have specialized knowledge, we sometimes ask for multiple approvals.

For example, for any non-trivial changes to distributed training, I'll probably ask for approvals from both @jmoralez (Dask knowledge) and one of @shiyu1994 and @guolinke (C++ and LightGBM internals knowledge).

[jameslamb]

These changes are included in this PR because I originally came to this page to add @borchero to the list of maintainers.

Explanation of the changes to these docs:

• none of this applies to only "Critical" issues, so removed language like that
• list of maintainers was seriously out-of-date
  • many of these people have not interacted with the repo in years
  • some of the notes about specialization were wrong (e.g. implying that I only work on R and Dask)
• example questions don't add much information above what we ask for in the issue templates (https://github.com/microsoft/LightGBM/tree/master/.github/ISSUE_TEMPLATE)

[jameslamb]

I'm basically proposing changing this from "list of all maintainers" to "who you should @ if you need timely help".