Add linter and apply lint fixes

[withinfocus]

Adds the Mega-Linter that I have found really helpful for keeping up with best practices. This uses its documentation "flavor" for content like this site has. As a result there's a bunch of existing fixes and configuration to perform.

[cloudflare-workers-and-pages]

Deploying 86-training with    Cloudflare Pages

Latest commit:	a81b208
Status:	✅  Deploy successful!
Preview URL:	https://5a906b3f.86-training.pages.dev
Branch Preview URL:	https://lint.86-training.pages.dev

View logs

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	1		0	0	0.07s
✅ COPYPASTE	jscpd	yes		no	no	1.68s
✅ CSS	stylelint	1		0	0	2.19s
✅ EDITORCONFIG	editorconfig-checker	24		0	0	0.08s
✅ HTML	djlint	3		0	0	1.15s
✅ HTML	htmlhint	3		0	0	0.32s
✅ JSON	jsonlint	1		0	0	0.18s
✅ JSON	prettier	1		0	0	0.32s
✅ JSON	v8r	1		0	0	2.94s
✅ MARKDOWN	markdownlint	15		0	0	0.8s
⚠️ MARKDOWN	markdown-table-formatter	15		1	0	0.29s
✅ REPOSITORY	checkov	yes		no	no	18.79s
✅ REPOSITORY	gitleaks	yes		no	no	0.56s
✅ REPOSITORY	git_diff	yes		no	no	0.0s
✅ REPOSITORY	grype	yes		no	no	33.34s
✅ REPOSITORY	secretlint	yes		no	no	1.02s
✅ REPOSITORY	syft	yes		no	no	2.32s
✅ REPOSITORY	trivy	yes		no	no	10.79s
✅ REPOSITORY	trivy-sbom	yes		no	no	0.14s
✅ REPOSITORY	trufflehog	yes		no	no	4.47s
✅ SPELL	cspell	25		0	0	3.37s
✅ SPELL	lychee	20		0	0	0.44s
✅ YAML	prettier	1		0	0	0.33s
✅ YAML	v8r	1		0	0	1.71s
✅ YAML	yamllint	1		0	0	0.34s

Detailed Issues

⚠️ MARKDOWN / markdown-table-formatter - 1 error

2 files contain markdown tables to format:
- content/command-based.md
- content/pid-basics.md

See detailed reports in MegaLinter artifacts
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff