docs: align the documentation with Thelia 3.0.0-beta3

[anoziere]

Brings the documentation in line with what 3.0.0-beta3 ships.

Install

• A GitHub token is now documented as a prerequisite. Composer reads the Thelia Flex recipes through the GitHub API; without a token Flex falls back on auto-generated recipes without reporting anything, and the install fails much later on You must either configure a "public_key" or a "secret_key". The page names the file to check afterwards.
• bin/install builds the front-office assets itself (importmap:install, then tailwind:build), so the manual npm step for the front theme is gone from every install page. The default-twig back-office keeps its Webpack Encore build, because its dist/ is not shipped, and that is stated where the step is asked for.
• PHP 8.3 and 8.4 are both supported.
• Version references moved to 3.0.0-beta3.
• install-reference lists the kernel phase as it really runs, including the JWT key pair and the asset build, and says why the two asset commands run last and when they are skipped.

Front-office theme

• The theme carries the front-office routes. Creating a theme documents the catch-all /{_view} controller that renders category, product, content and folder pages, why its priority is negative and why admin and api are excluded. The thelia/front-module package is no longer involved.
• The virtual product download route is documented: the theme dispatches VIRTUAL_PRODUCT_ORDER_DOWNLOAD_RESPONSE and never reads the file, and a shop with no virtual product module gets a 404 rather than a 500.
• config/views.yaml is documented, with the rules the core applies: optional file, nearest declaration wins, a request naming an internal view gets a 404 while a controller rendering the same template is unaffected.
• The Flexy asset pages describe AssetMapper and Tailwind instead of Webpack Encore: importmap.php, assets/, asset() and importmap(), tailwind:build --watch, and no <assets> tag in template.xml.
• The PHP 8.3 case for symfony/ux-twig-component is explained, along with the extension that supplies provide() and inject() when the 2.x line is resolved.

Two 3.0 pages that the previous commits changed only under docs/ are brought back in sync.

Not covered

The Flexy pages still describe the component layout, the Twig namespaces and the SEO helpers of the previous theme. That drift predates this pull request and needs its own pass.