feat(seo): wire internal links between the 17 blog posts and the pages that sell

The blog was an island: zero post→page links to hosting/upgrade/support/courses,
and only one page→post link in the whole site. Posts ranked for nothing while the
old blog host ranked for our long-tail queries.

Posts → pages (26 new in-body contextual links across 17 posts):
each post now carries one or two links to the page that answers its reader's next
question — odoo-hosting-2027, odoo-community-upgrade, support, the workshops hub,
the n8n masterclass, and the plan pricing on index.html#pricing.

Pages → posts (7 pages, one contextual link each):
index.html (OCA section), odoo-hosting-2027, odoo-community-upgrade, support,
workshops/, odoo-n8n-masterclass/, odoo-erp-automation-bootcamp/ now link into the
post that fits their own subject. The clearest miss is fixed — the
community-vs-enterprise-cost post and the odoo-community-upgrade page now link
to each other.

Anchors wrap existing sentences where the text already said the thing; four short
linking sentences were added (index OCA section, support first-steps FAQ, masterclass
lessons, bootcamp included section). No new components, no related-posts widget,
in-body anchors only. English pages only; DE/ES untouched (translator follow-up).

blog/posts/howto-custom-module.html

@@ -227,8 +227,8 @@
                 <div class="post-content">
                     <p>
                         One of the most powerful features of Odoo is its modular
-                        architecture. With ODOO4projects' On the Rise plan and
-                        above, you can install custom modules directly from Git
+                        architecture. With ODOO4projects' <a href="../../index.html#pricing">On the Rise plan and
+                        above</a>, you can install custom modules directly from Git
                         repositories. Here's how.
                     </p>
                     <h2>Step 1: Connect Your Git Repository</h2>