SRU docs added to navig. structure

[rkratky]

Description

This PR adds the imported (#228) SRU docs into the navig. structure of the Project docs while mostly preserving the structure of the orig. docs set.

• SRU explanation & reference articles added to "How Ubuntu is made -> Process overviews -> Stable Release Updates (SRU)"
• SRU howto articles added to "Contributors -> Stable Release Updates"
• The Diataxis grid and the generic "Project and community" section were removed from the SRU landing page.

The only change is that the landing page for SRU explanation articles was merged with the SRU docs main landing page. This new landing page is now found at "How Ubuntu is made -> Process overviews -> Stable Release Updates (SRU)".

A link to an existing intro article (Introduction to SRU) was also added to the landing page for the howto articles: "Contributors -> Stable Release Updates".

Preview build: https://canonical-ubuntu-documentation-library--253.com.readthedocs.build/project/

Next steps

Following the merging of this PR, redirects will be put in place from the orig. docs set, so that all existing links and bookmarks pointing to the docs at documentation.ubuntu.com/sru would continue to work.

---

Checklist

• I have read and followed the Ubuntu Project contributing guide
• My pull request is linked to an existing issue (if applicable)
• I have tested my changes, and they work as expected

---

Additional notes

@basak, your invitation to the repo is still pending, so the CODEOWNERS system is not automatically triggering a review request for you. If you add a comment to the PR, I should be able to request a review from you manually.

[s-makin]

Just two things I've spotted, both non-blocking and can be fixed in a follow-up:

• In SRU/stable-release-updates and SRU/explanation/pipeline the diagrams don't render.
• In SRU/reference/package-specific there are multiple h1-level headers which introduces extra noise into the navigation menu. Not sure if the original intention was for them to be surfaced at the top level (implying they should be on different pages) or if they should indeed be subsections under the "package specific notes" page (implying they should be h2-level).

[rkratky]

Just two things I've spotted, both non-blocking and can be fixed in a follow-up:

In SRU/stable-release-updates and SRU/explanation/pipeline the diagrams don't render.

That's weird. It works ok on local :/ Looking into it.

In SRU/reference/package-specific there are multiple h1-level headers which introduces extra noise into the navigation menu. Not sure if the original intention was for them to be surfaced at the top level (implying they should be on different pages) or if they should indeed be subsections under the "package specific notes" page (implying they should be h2-level).

I believe that's intentional. Those are indeed independent pages.

[rkratky]

• In SRU/stable-release-updates and SRU/explanation/pipeline the diagrams don't render.

Fixed. Thanks for the heads up.

[panlinux]

+1