fix: automatic routing for translated text routes

[stephanegigandet]

This PR improves the routing of text pages so that translated text routes in .po files (e.g /discover and /conditions-d-utilisation in French) are matched with the corresponding /lang/fr/discover.html text.

Note: openfoodfacts-web currently has links or translated file names ( https://github.com/openfoodfacts/openfoodfacts-web/tree/main/lang/fr/texts/ ). Those should be removed.

This PR replaces #12644

Also renamed Index.pm to Texts.pm (clearer) and added some docs and a couple of tests.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.77778% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.77%. Comparing base (b813b7a) to head (ba5c78f).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
lib/ProductOpener/Texts.pm	61.33%	23 Missing and 6 partials ⚠️
lib/ProductOpener/Routing.pm	24.13%	20 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12651      +/-   ##
==========================================
+ Coverage   46.40%   51.77%   +5.37%     
==========================================
  Files          95       96       +1     
  Lines       24728    24844     +116     
  Branches     5922     5953      +31     
==========================================
+ Hits        11474    12863    +1389     
+ Misses      11700    10255    -1445     
- Partials     1554     1726     +172     

Flag	Coverage Δ
integration-test-group-1	31.29% <52.33%> (?)
integration-test-group-2	27.17% <52.33%> (?)
integration-test-group-3	8.91% <61.03%> (?)
integration-test-group-4	10.85% <62.33%> (?)
integration-test-group-5	20.05% <52.33%> (?)
integration-test-group-6	24.91% <52.33%> (?)
integration-test-group-7	10.41% <62.82%> (?)
integration-test-group-8	8.92% <61.03%> (?)
integration-test-group-9	9.12% <62.33%> (?)
unit-test-group-1	26.41% <51.88%> (+0.06%)	⬆️
unit-test-group-2	21.89% <62.82%> (+0.07%)	⬆️
unit-test-group-3	20.59% <51.88%> (+0.09%)	⬆️
unit-test-group-4	31.21% <52.33%> (-0.05%)	⬇️
unit-test-group-5	20.18% <62.82%> (-0.17%)	⬇️
unit-test-group-6	29.45% <50.94%> (+0.17%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud