chore: update links from developers.google.com to docs.blockly.com

[maribethb]

The basics

• I validated my changes

The details

Resolves

Proposed Changes

• Replaces most links to developers.google.com with their equivalent blockly.com links.
• Updated a small number of absolute URLs within the docs site to relative ones so that they get checked by the broken link checker in docusaurus

Not changed:

• The 404 occurrences of https://developers.google.com/blockly/xml. This is
  Blockly.utils.xml.NAME_SPACE, the XML namespace identifier used in
  serialization and every .xml test fixture, not a link. Can't change the namespace without breaking xml deserialization.
• packages/blockly/msg/json/*.json (synced from TranslateWiki). The hand-written
  README.md in that directory was updated however.
• packages/blockly/demos/storage/index.html, which links to a Cloud Storage page
  that has no equivalent on the new site. We'll delete this soon anyway.
• CONTRIBUTING.md because I have a separate PR that does that.

Reason for Changes

The canonical docs site is now https://docs.blockly.com.

Test Coverage

Claude generated this change by:

1. creating a resolver that took old links and routed them through the docs/static/_redirects paths listed.
2. accounting for the url fragment format which changed in the move and also depends on the heading names which have sometimes changed
3. ensured that the final url actually existed and that the destination reflected what was supposed to be linked to, since in some cases the content itself moved as well, not just the urls.
4. flagged any problematic urls to me so i could provide input on a suitable replacement

I reviewed everything claude flagged as a judgment call and spot-checked the rest of it.

Documentation

this is that

Additional Information

Judgment calls where no clean one-to-one mapping existed:

• get-started/web#configuration → /guides/configure/configuration_struct/#the-options-dictionary.
  The redirect points at get-the-code/, which has no configuration section; the
  options dictionary now lives on its own page.
• "Block change events" → /reference/blockly.events_namespace.blockchange_class/.
  The old anchor has no successor, and the guide's #event-types section is a stub
  that defers to the reference docs.
• "event listeners" → /guides/configure/events/#listen-to-events-from-the-workspace
  (the addChangeListener section).
• Block Factory demo helpUrl → legacy-blockly-developer-tools/, since the tool
  doing the linking is the legacy tool.
• define-blocks#block_inputs → define/block-anatomy/#inputs; the inputs content
  moved to the anatomy page, not to block-definitions.
• documentation.yaml "developer site" → the docs home, rather than the redirect's
  "What is Blockly?" target, since the sentence refers to the site as a whole.
• In generator.ts the URL sits inside a runtime TypeError string, so it stays
  schemeless and drops the fragment — it's read in a console, not clicked.

[zspriggs]

Looks good to me!