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

[lindangulopez]

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.

[meldra]

PR #130 has variations of some of the requested changes.

1. Instead of pointing at the generator directly, I've pointed at the tools page which includes the generator but also tools using the mappings, so much more topical to the context.
2. Something like this added as a tip at the very bottom of the shortcode.
3. This will be kept for when the user guide is touched again, which is not that PR.