Neovim plugin: migrate troubleshooting page

What does this MR do?

After !162478 (merged) copied the main README from the Neovim plugin project into the gitlab repo, let's go get the next most useful page - the troubleshooting page.

This MR takes the content of that page, and:

• puts it in a new page (doc/editor_extensions/neovim/neovim_troubleshooting.md)
• reformats to adhere to GitLab tone and style
• adds in the new troubleshooting info from GCS:unavailable in a simple go file (gitlab-org/editor-extensions/gitlab.vim#122)

I'll need to follow up with a left-nav entry.

Related issues

• Related to Neovim extension: move README contents to this ... (!162478 - merged) where I moved the README
• Related to GCS:unavailable in a simple go file (gitlab-org/editor-extensions/gitlab.vim#122) where the GCS:unavailable error was reported
  • Related to Neovim: stub out the 'remote origin' requirement (!161255 - merged) where I've proposed adding a setup step to help future users avoid this problem
• Related to https://gitlab.com/gitlab-org/editor-extensions/meta/-/issues/110+ which contains the larger context of where the extension docs should live

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.