docs: splitting readme into pebble docs with the diataxis method

[IronCore864]

Splitting the README documentation into smaller chunks into the docs folder, using the Diataxis method.

Click here to see the preview..

Major changes:

• custom config (site title, links, etc.)
• created a grid layout on the home page, thanks to @AnneCYH
• Splitting the README into smaller chunks following the Diataxis method. No content change except one small item: Some paragraphs use linebreaks while others don't, for consistency, all paragraphs are converted to the same style without linebreaks.

TODO: The current tutorial (docs/tutorial/getting-started.md) is too long, maybe we need to split it into smaller chunks, some fit into the "how-to" category while others may fit into the "explanations" more.

[AnneCYH]

Hi @IronCore864 , this is a lot clearer, thanks!

2 changes for now:

1. "How-to guides" index title should be sentence case.
2. Remove the "Table of contents" from each index page. The no-stacked-headings rule.

I can see the content starting to modularise, which will make it easy to keep each section clean, avoid duplicates, and easy to cross-reference.

I do think some sections should be merged or added.

1. Get logs + use log forwarding are all related to a single entity for Pebble, "logs", and don't need to be separate either.
2. Other content that may be 'missing', e.g. related to Files. When i run pebble help, the commands are categorised, but the current README doesn't necessarily address "Info" (e.g. managing your Pebble installation in terms of version, updating to a newer version), or "Files". But this can be done in a separate PR.

Overall I think it's a good start. I only left 1 minor comment about the title for the How-to index page; but other than that I think it's good to go for the first draft.

Thanks!