chore(docs): use a builder-portable homepage check in the override template

[lesnik512]

Why

page.is_homepage is a MkDocs Page property with no equivalent in Zensical, which this org is
evaluating as a future docs builder (#60). It fails silently, not loudly: at Zensical 0.0.59 the
property does not exist anywhere in the source, and an undefined name resolves falsy in MiniJinja,
so the home page would fall through to the non-home branch. Here that means a doubled
<title> ("Modern Python - Modern Python", the exact bug the comment in this file describes) plus a
wrong og:title / twitter:title, with no warning.

nav.homepage is exposed by both builders: MkDocs documents it as homepage: Page | None
(https://www.mkdocs.org/dev-guide/themes/), and Zensical serializes it in
crates/zensical/src/structure/nav/view.rs. Comparing page.url against it is portable.

Part of #71. The sibling PRs are modern-python/faststream-outbox#167,
modern-python/modern-di#455 and modern-python/lite-bootstrap#170.

Design

{%- set is_home = nav.homepage and page.url == nav.homepage.url %} once per block, then is_home
where page.is_homepage used to be. Both blocks need their own set; Jinja block scopes do not
share it.

• The nav.homepage and guard is load-bearing: MkDocs types it Page | None. When it is None the
  expression is falsy, matching is_homepage being False on every page of a site with no home
  page.
• The {%- left-trim is also load-bearing: without it the new tag adds a blank line to every
  rendered page.

The explanatory comment above htmltitle still describes the situation accurately and is unchanged.

Non-goals

Not a Zensical migration. #60 stays open and stays blocked on exclude_docs and
validation.omitted_files. This is a no-op under MkDocs today and does not depend on that work.

Verification

uv run mkdocs build --strict on main and on this branch, output diffed recursively: identical.

This site is a single page, so that diff alone would only exercise the homepage branch. I added a
temporary second page for the comparison and confirmed both branches match:

	main	this branch
home <title>	Modern Python	Modern Python
non-home <title>	Scratch Verify Page - Modern Python	Scratch Verify Page - Modern Python
non-home og:title	Scratch Verify Page · Modern Python	Scratch Verify Page · Modern Python

The temporary page is not part of this diff.