Add cross-link to and from Crosswalk landing page to User Guide and Generator #128

1. Crosswalk landing page (/crosswalk/)

Summary: the Crosswalk landing page (/crosswalk/) doesn't link to the User Guide or the Generator
anywhere outside the top nav, so someone arriving with a "what is CodeMeta" or "just let me
make a file" need has no way to redirect from this page.

Proposed change: add directly under the intro paragraph, before "Contribute!":

• Looking for a specific field mapping while filling out a CodeMeta file? The
• User Guide explains the core CodeMeta concepts (JSON-LD, nesting, roles),
• and the CodeMeta Generator can build a file for you directly, Crosswalks are
• most useful once you already have metadata in another format and want to see how it maps.

Why: related to #112 (crosswalk sync) but independent of it, this closes on its own regardless
of how that sync work lands.

2. Individual crosswalk pages (e.g. /crosswalk/github/, /crosswalk/spdx-2-3/, etc.)

Summary: individual crosswalk pages don't link back to the User Guide or Generator either,
the same fragmentation as the landing-page gap above.

Proposed change: since these pages are generated/synced from crosswalk.csv, add one line
to the shared template rather than editing each page by hand:

• New to CodeMeta? See the User Guide for how these terms are used in a
• codemeta.json file, or jump straight to the Generator.

Why: related to #112. This one depends on the sync mechanism from that issue, so it may
land later than sections 1 and 3 here, noting it as its own item so it doesn't block them.

3. User Guide, Context section

Summary: the User Guide's Context section explains @context and links to schema.org
and the Developer Guide, but never points to the Crosswalk directory, the natural next stop
for "does CodeMeta already cover the terms I use elsewhere?"

Proposed change: add after the existing paragraph on extending the context:

• If you're migrating metadata from another format (e.g. a package manager manifest or an
• existing metadata standard), check the Crosswalk directory first, it may
• already map your source format's fields to CodeMeta terms.

Why: related to #112. Independent of the crosswalk sync mechanics, so it can close
regardless of how #112 or section 2 above land.