Re-organize board-specific samples per vendor and add zephyr:code-sample directive

[kartben]

• Added missing .. zephyr:code-sample:: directives
• Grouped samples by vendor
• Added intermediate table of contents to get rid of the very flat/messy top level ToC we had

Note 1: The size of the change set is impressive, but it really is mostly just moving files around while adding proper HTML redirects when necessary. Splitting this into multiple PRs would lead to mostly systematic merge conflicts in the redirects.py file.

Note 2: samples/boards/arc_secure_services probably needs a better home as it's not clear it's a "board" sample. I have left it out from the PR and this is the only remaining "non-vendor" top-level folder underneath samples/boards.

BEFORE: https://docs.zephyrproject.org/latest/samples/boards/boards.html
AFTER: https://builds.zephyrproject.io/zephyr/pr/78089/docs/samples/boards/boards.html

[kartben]

@mmahadevan108 thanks for spotting this needed to be rebased :) done!

[marekmatej]

Espressif parts LGTM! Thanks for this helpful update!

[kartben]

This has had approvals (either in current version of the PR on earlier version before a rebase was needed) from all the major vendors impacted by the change. I will be re-assigning to myself as this is in the samples area at the end of the day, and to make sure this can be merged sooner than later as it is likely to generate merge conflicts. Thanks everyone for the reviews!