Make template docs the default tab, and change the order of tabs

[LucidDan]

Description

This does three things:

1. Changes order of tabs so "Docs" comes first, "Source" comes last (effectively swapping these two tabs).
2. Makes the "Docs" tab the default tab.
3. Adds a "No template doc available" message if there is no .md file or the file is empty.

Number 3 is not part of the original request but I think it makes sense, I'd like a review of that thought - my line of thinking was that "template source" is always set to something, so you never get an empty tab. Adding a message makes it clear why there is nothing there for the docs. If you don't agree, I committed the change separately so its easy to revert it.

Note: I have not as yet screenshotted a replacement for docs/images/pattern-library-screenshot.webp and docs/images/getting-started/getting-started-complete.png, but if needed I can do that before the PR is merged.

Fixes #259

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

[helenb]

I've tested this and it is working well - thank you @LucidDan. Just one small nitpick which is not essential.

[thibaudcolas]

thank you!