Add mention of demo server in translation docs

[gonzalo-bulnes]

Status

Ready for review

Description of Changes

• Add mention of the demo server in the translation docs, in the context of gathering context about a source string.
• Add a how-to guide to use the SecureDrop's demo server as a source or a journalist.

Note: For consistency, I made the how-to as detailed as the current version of the Weblate how-tos. That means that a few UI element names are included in the instructions, which in turn introduces some coupling between this section of the documentation, the demo server home page and (to a minor extent) the Journalist Interface login form.

Fixes #167
Closes #117

Testing

• Proof-reading cannot hurt.
• Check the link name in the demo instance page
• Check the credential names in the demo instance page and Journalist Inteface login form

Release

No special considerations for release.

Checklist (Optional)

• Doc linting (make docs-lint) passed locally
• Doc link linting (make docs-linkcheck) passed
• You have previewed (make docs) docs at http://localhost:8000

[eloquence]

LGTM! :) Made one small tweak to clarify that any browser can be used to access the demo.