docs(UPIIS-47): serve UPI Issuance images from CDN + tighten API field validation

[Anindya-Pandey]

What

Fixes broken images in the UPI Issuance docs and tightens field-validation
detail in the API tables.

Images

The UPI Issuance pages referenced diagrams by local path
(<img src="/upi-issuance/onboarding-flow.png">). Those paths only resolve in
the docs-mdx preview app — on the published docs site they render as broken
images.

Uploaded the five diagrams to the assets bucket following the folder structure
in the README, and repointed all 7 refs at the CDN:

Image	URL
onboarding-flow.png	https://docs-assets.setu.co/latest/payments/upi-issuance/onboarding-flow.png
device-change-flow.png	.../payments/upi-issuance/device-change-flow.png
vpa-activation.png	.../payments/upi-issuance/vpa-activation.png
user-activation.png	.../payments/upi-issuance/user-activation.png
device-binding.png	.../payments/upi-issuance/device-binding.png

All five verified live — HTTP 200, Content-Type: image/png, byte sizes
match the source files.

Validation docs

user-otp.mdx and device-binding.mdx tables now carry explicit field
constraints instead of prose — mobile as 12 digits (^[0-9]{12}$), otp as
6 digits (^[0-9]{6}$), vpa as prefix@handle (max 255), deviceId and
idempotencyKey length caps — and the invalid-request rows say what actually
fails validation.

Testing

• curl against each CDN URL → 200 image/png, sizes match source
• No local src="/upi-issuance/..." refs remain in content/

Notes for reviewers

• Assets went to docs-mdx-assets, but the README documents the bucket as
  strapi-assets. The CDN serves from docs-mdx-assets, so the README name
  looks stale — worth a follow-up fix so the next person isn't sent to the
  wrong bucket.
• Existing assets in the bucket use a flat legacy Strapi layout
  (latest/<Asset_Name>/<Asset_Name>.png). This PR follows the documented
  convention instead (mirroring the /content folder structure), which is what
  the README prescribes.

[github-actions]

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• Created a folder with the same name, if any children were to be added to the page
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• Add frontmatter with all the metadata
• Review the order of items in Sidebar using the Sidebar preview option
• Add API reference in JSON format (OpenAPI or Swagger) into created .json file.
• Used the Content preview portal to view changes
• Once all changes are done, update the menu items by using the Menu Items option