docs(web-bundles): fix install framing and add update/customize guidance (#2423)

- Drop "one-click install" framing in README and explanation page; setup is
  multi-step but consistent across the shelf.
- Drop "two files (sometimes three)" claim; honestly name SKILL.md,
  INSTRUCTIONS.md, and any required data files (templates, CSVs,
  validation checklists).
- Add explicit setup pattern in README (create Gem/GPT, upload knowledge,
  paste instructions, save).
- Add "Updating and customizing" section to the explanation page covering
  re-upload-the-attachments updates and the rule of thumb that custom
  changes belong in the pasted instructions block, not the knowledge
  files, so future updates don't clobber team customizations.