docs: enhance how-to navigation with categories (Phase 3)

Reorganize how-to guides with clear categories and popular guides section
for improved discoverability.

Changes:
- Add "Popular Guides" section with links to most common tasks
- Categorize guides into logical groups:
  * Getting Started: containers, bootstrap-constraints
  * Building Packages: git repos, repeatable builds, parallel, web server
  * Build Configuration: pyproject overrides, multiple versions, pre-releases
  * Analyzing Builds: graph commands
- Add descriptive text explaining guide categories
- Add "See Also" section linking to customization and reference docs

Impact: Users can quickly scan categories and find relevant guides

Related: #969

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: dhellmann

docs/how-tos/index.rst

@@ -1,9 +1,67 @@
 How-tos
 =======
 
+Task-oriented guides for common workflows and customization scenarios.
+
+Popular Guides
+--------------
+
+Quick links to frequently used guides:
+
+* :doc:`containers` - Running fromager in containers (recommended approach)
+* :doc:`bootstrap-constraints` - Pin versions for reproducible builds
+* :doc:`graph-commands/using-graph-why` - Debug unexpected dependencies
+* :doc:`repeatable-builds` - Ensure build reproducibility
+
+Getting Started
+---------------
+
+Essential guides for initial setup and first builds.
+
+.. toctree::
+   :maxdepth: 1
+
+   containers
+   bootstrap-constraints
+
+Building Packages
+-----------------
+
+Guides for building packages from various sources and configurations.
+
+.. toctree::
+   :maxdepth: 1
+
+   build-from-git-repo
+   repeatable-builds
+   parallel
+   build-web-server
+
+Build Configuration
+-------------------
+
+Customize builds with overrides, variants, and version handling.
+
 .. toctree::
    :maxdepth: 1
-   :glob:
 
-   *
-   */index
+   pyproject-overrides
+   multiple-versions
+   pre-release-versions
+
+Analyzing Builds
+----------------
+
+Understand and debug dependency graphs and build issues.
+
+.. toctree::
+   :maxdepth: 1
+
+   graph-commands/index
+
+See Also
+--------
+
+* :doc:`/customization` - Comprehensive guide to customization options
+* :doc:`/reference/config-reference` - Configuration reference
+* :doc:`/reference/hooks` - Override plugins and hooks