feat: add Ubuntu package archive overview diagram

[dviererbe]

This commit adds a mermaid diagram that gives a
quick overview over the structure and terminology
of the Ubuntu package archive.

note: This diagram is a copy from the Ubuntu server
documentation. See the Origin footer below.

This commit also adds a sphinx mermaid extension
to render the mermaid diagram.

todo: The diagram has a low contrast in dark mode.

Origin: https://documentation.ubuntu.com/server/tutorial/managing-software/#where-do-packages-come-from
Fixes: #82

[prash813]

Yes the diagram looks very informative and makes relation among different terms much more intutitive

[dviererbe]

note: The linkcheck failures can be ignored

(explanation/development-process: line   60) broken    https://discourse.ubuntu.com/c/release/ - 404 Client Error: Not Found for url: https://discourse.ubuntu.com/c/release/
(reference/glossary: line 1318) broken    https://launchpad.net/~ubuntu-esm-team - 404 Client Error: Not Found for url: https://launchpad.net/~ubuntu-esm-team

They are already fixed in PR #87

[tsimonq2]

Hey @dviererbe! A few notes:

• Splits into components should somehow make it more clear that all of those pockets have those components, not just -updates.
• -release is a hidden pocket; only Britney knows that it exists. I'd emphasize that somehow.
• Some more unique colors would be great, perhaps a bit of Ubuntu Orange mixed with different colors.

Great start, thanks!

[dviererbe]

@tsimonq2

• Splits into components should somehow make it more clear that all of those pockets have those components, not just -updates.

note: I fell like the pre-text is pretty clear on that:

ubuntu-packaging-guide/docs/explanation/archive.rst

Lines 17 to 21 in 2a4291b

	Let's have a quick overview with this diagram. The general flow is that the
	Archive splits into :ref:`Ubuntu series <ArchiveSeries>`. Each series is split
	up into :ref:`pockets <ArchivePockets>`, and then each pocket contains four
	:ref:`components <ArchiveComponents>`. If we tried to show all of this on one
	diagram, it would be quite extensive, so let's take a look through a single path:

• Some more unique colors would be great, perhaps a bit of Ubuntu Orange mixed with different colors.

note: We should include that in the scope of a new issue that also fixes the low contrast in dark mode, I mentioned above. I do not want to block the PR on a styling issue, because they can take a lot of time to get right, especially because they can trigger longer discussions with the documentation team, because they want to achieve a consistent style.

• -release is a hidden pocket; only Britney knows that it exists. I'd emphasize that somehow.

question: I am not 100% sure what you mean by that. Do you refer to the fact that the release pocket is actually just $SERIES instead of $SERIES-release? I can very well see the release pocket in the archive and apt is also configured by default with the release pocket as a source.

note: this is documented in the suites section

ubuntu-packaging-guide/docs/explanation/archive.rst

Lines 168 to 185 in 2a4291b

	Suite
	-----
	A combination of a series and a pocket. For example:
	+---------------------+----------------------+---------------------------------+
	| Suite | Series | Pocket |
	+---------------------+----------------------+---------------------------------+
	| ``jammy`` | ``jammy`` | :ref:`ArchivePockets_Release` |
	+---------------------+----------------------+---------------------------------+
	| ``jammy-security`` | ``jammy`` | :ref:`ArchivePockets_Security` |
	+---------------------+----------------------+---------------------------------+
	| ``jammy-updates`` | ``jammy`` | :ref:`ArchivePockets_Updates` |
	+---------------------+----------------------+---------------------------------+
	| ``jammy-proposed`` | ``jammy`` | :ref:`ArchivePockets_Proposed` |
	+---------------------+----------------------+---------------------------------+
	| ``jammy-backports`` | ``jammy`` | :ref:`ArchivePockets_Backports` |
	+---------------------+----------------------+---------------------------------+