[Docs] Merge Diataxis reorg feature branch into main

[stichbury]

Description

Over a period of several weeks, @rashidakanchwala and I have reorganised the Configuration and Data Catalog docs to fit the Diataxis pattern (concept pages plus separate how-to pages), and I have tidied the top-level nav so section labels read as topics rather than imperative verbs.

• Configuration (Reorganize Configuration docs following Diataxis framework #5513, [Docs] Rename config explanation docs filenames (back to original names) #5620): retired parameters.md, credentials.md, advanced_configuration.md, and parameter_validation.md. New concept pages: parameters_and_credentials.md, templating.md. New how-to pages: how_to_configure_project.md, how_to_use_parameters_and_credentials.md, how_to_use_templating.md.
• Data Catalog ([Docs] Reorg datacatalog docs for Diataxis framework #5622): trimmed the concept pages, extracted how-tos (how_to_configure_the_data_catalog.md, how_to_use_dataset_factories.md, how_to_use_partitioned_and_incremental_datasets.md, how_to_access_the_data_catalog_in_code.md), added an introduction landing page.
• Nav labels: Configure → Configuration, Develop → Development, Deploy → Deployment, Extend → Extend a project, Create → Create projects, Catalog Data → Data Catalog, Build → Nodes & Pipelines; the one-page Inspect section folded into Development.
• Plus a handful of child-label tweaks — most usefully Programmatic usage → Kedro sessions, Reusing pipelines (namespaces) → Namespaces, and dropping redundant Kedro / How to prefixes.
  All file removals and renames are covered by existing redirect_maps entries in mkdocs.yml.

Development notes

Rebuilds cleanly and tested by visual inspection. The built docs are here.

Developer Certificate of Origin

We need all contributions to comply with the Developer Certificate of Origin (DCO). All commits must be signed off by including a Signed-off-by line in the commit message. See our wiki for guidance.

If your PR is blocked due to unsigned commits, then you must follow the instructions under "Rebase the branch" on the GitHub Checks page for your PR. This will retroactively add the sign-off to all unsigned commits and allow the DCO check to pass.

Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team