Add canonical examples for the marketing SDK pages

[Jonesy464]

Part of anvilco/marketing-site#3646 (SDK repos own their code examples; the central anvil-api-usage-examples repo is being sunset).

Adds examples/{fill,generate-markdown,generate-html,create-etch-packet}.{js,ts}: the canonical, display-ready examples the marketing SDK pages will source at build (the sourcing shipped with anvilco/marketing-site#3617 scans exactly this directory; the kebab-cased basename becomes the example concept, and the file extension routes it to the JavaScript or TypeScript page).

Content is migrated from anvil-api-usage-examples with a refresh pass. Fixes found in the sources:

• The e-sign example shipped baseURL: 'https://staging.useanvil.com', pointing anyone who copied it at staging. Removed.
• Its NDA-upload half referenced a static/ asset that only exists in the central repo; the example is now the self-contained sample-template flow (upload-with-fields stays documented in the API docs).
• The fill example's placekitten.com image URL is a dead service. Dropped.
• font-style: underline in the HTML example is invalid CSS; now italic.
• Replaced the central repo's ../lib/run harness with plain async entrypoints, and trimmed headers. The run commands are framed for a consumer project since @anvilco/anvil can't self-resolve inside this repo (no exports field).

Verification:

• All four TS files type-check against the published @anvilco/anvil under both moduleResolution: bundler and node/CommonJS; JS mirrors pass Node ESM syntax checks.
• All four executed live against the API with a dev key: fill and both generates returned 200 with valid PDFs (confirming the sample template and its field IDs are current), and the e-sign mutation created a packet (run as an isTest draft so no email was sent; the committed file keeps isDraft: false).
• The marketing site's deriveLanguageExamples resolves this directory to exactly the four concepts per language.

[fostimus]

can you add allowUpdates: false in here? it powers interactive signing, which a lot people want/will need. the comment can be about how you need it set to true to use interactive signing, and interactive signing is a product pack feature

[Jonesy464]

Added to both the .js and .ts examples:

// Set to true for interactive signing, where signers fill fields as they
// sign. Over the API that needs the Product pack or Enterprise plan
allowUpdates: false,

Used "Product pack or Enterprise" since that's what the e-sign doc says for API access.

One catch: the Python and C# examples can't have this field. allowUpdates isn't on CreateEtchPacketPayload in python-anvil or Payloads/Request/CreateEtchPacket.cs in dotnet-anvil. node-anvil works because it passes variables straight through.

Those two SDKs are missing a lot more than this one. createEtchPacket takes 25 args, python-anvil is missing 9 and dotnet-anvil 12:

• python-anvil: allowUpdates, requireSignatures, finishPageOptions, excludeCertificateFromSignerDownloads, signatureRecipients, signatureProvider, advancedCreate, detectBoxesAdvanced, organizationEid
• dotnet-anvil: those plus enableEmails, createCastTemplatesFromUploads, duplicateCasts

Should I create an issue to update them?