Remove obsolete hackathon-era content from public docs

[swarna1101]

The site still carries hackathon-era content. PR #40 already removed the CLI, Docker and experiment guides, but their URLs now return 404.

Remove

• "Quick start (HOP)" sidebar link in .vitepress/config.mts. HOP is a Docker Compose experiment and dev-setup framework, not the operator path.
• The commented-out "Flexnode API Docs" block in .vitepress/config.mts.
• docs/how-to-guides/overview.md, an "Under Construction" stub.
• docs/learn/opt.md, a stub for the MUM token.
• docs/learn/how-to-stake-mum.md, a stub.
• docs/learn/overview/intro.md: the Flexnodes section, the "permissionless network of flexnodes" claim, and "Intended users". Issue Write Start here and Part 1: Getting in #48 replaces this page.

Redirects (vercel.json)

• /docs/guides/* to Start here.
• /docs/how-to-guides/*, /docs/learn/opt and /docs/learn/how-to-stake-mum to Start here.

Done when

None of these pages are live, and every removed URL redirects instead of returning 404.

[linear-code]

OPT-1113