Fix long-standing markdownlint alerts

[spier]

Fixing these issues that were reported by markdownlint for a while now.

This lead to a failing check (red cross) or almost all PRs, which is confusing for contributors (and annoying for maintainers :)).

Error: book/ja/toc_template.md:97 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]
Error: book/ja/toc.md:97 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]
Error: patterns/1-initial/innersource-portal-hygiene.md:30:10 MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]
Error: patterns/1-initial/innersource-portal-hygiene.md:31:10 MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]
Error: patterns/1-initial/walk-the-innersource-talk.md:167:99 MD047/single-trailing-newline Files should end with a single newline character
Error: patterns/2-structured/project-setup/README.md:3:36 MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]
Error: patterns/2-structured/release-process.md:50:55 MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]
Error: translation/README.md:52:217 MD059/descriptive-link-text Link text should be descriptive [Context: "[here]"]