Docs: improve Issue Boards overview (to deprecate feature page)

What does this MR do?

• Make Issue Boards doc more "enticing" to deprecate the feature page https://about.gitlab.com/features/issueboard/
• Selectively bring in any useful visuals/information from the existing Feature page on about.gitlab if one exists
• Good summary/intro
• Selectively add any good YouTube video and/or blog link we may already have
• Easy-to-understand diagrams, videos, etc.
• Add content from the feature page to make the feature overview more "enticing"
• Reorganize document headings for a better reading flow
• Update screenshots
• Reorganize EE-only features' sections
• Display features first, how-tos later
• Add missing EE-only features to the CE doc (for consistency, all should be here, or none of them)

Review app: http://review-docs-docs-t7n8dm.178.62.207.141.xip.io/ce/user/project/issue_board.html

Related issues

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50503
• For &305 (closed)

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7483

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7483), CE-MR afterwards