Add configurable ignored selectors to exclude HTML elements from processing

Problem/Motivation

When Content First renders an entity, the output can include navigational and administrative HTML elements that are noise for downstream consumers such as LLMs or documentation tools. Elements like <nav class="pager"> or <ul class="contextual-links"> have no content value and pollute the generated markdown.

There is currently no way to exclude specific elements from processing without patching the module.

Proposed resolution

Add an ignored_selectors configuration key that holds a list of CSS selectors. Before any tag stripping or markdown transformation, elements matching those selectors are removed from the rendered HTML — including their children.

A new static utility class HtmlSelectorRemover handles the removal using Drupal\Component\Utility\Html and Symfony\Component\CssSelector\CssSelectorConverter, following the same pattern as HtmlTagTransformer.

The settings form exposes a textarea (one selector per line) so site administrators can manage the list without deploying code.

Default selectors shipped with the module:

• nav.pager — pagination navigation
• ul.contextual-links — Drupal inline admin edit links

Remaining tasks

• Review and commit

User interface changes

A Ignored selectors textarea is added to the Content First settings form at /admin/config/content/content-first. Each line is treated as one CSS selector. The field ships pre-populated via config (not hardcoded in PHP).

API changes

New class Drupal\content_first\HtmlSelectorRemover with a single public static method:

HtmlSelectorRemover::remove(string $html, array $selectors): string

Data model changes

A new key ignored_selectors (sequence of strings) is added to content_first.settings. Existing sites are migrated via content_first_update_20003(), which sets the default value of ['nav.pager', 'ul.contextual-links'] if the key is not already present.