docs: Cut (mostly) CI/CD info from README

Description

Yet another round of cuts to the README as I get info moved over to the gitlab repo. This MR truncates these sections:

• Information about your branch - pipelines, MR, closing issue
• Commands (I should have cut this in a previous MR but didn't. No idea why.)
• Extension settings
• Status bar: details
• Validate GitLab CI/CD Configuration
• Show merged GitLab CI/CD configuration
• CI/CD variable autocompletion

It's the companion to gitlab!172917 (merged), which adds this information into the gitlab repo, mostly into a new CI/CD page.

Related Issues

• Depends on VS Code: create CI/CD page (gitlab!172917 - merged) being merged first
• Related to Add CI/CD management page to VS Code section (gitlab-docs!5209 - merged) which will add a new page to the GitLab docs site global nav
• Related to Plan for migrating VS Code user documentation (#1530 - closed) which I continue to slowly chip away at

How has this been tested?

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes