update architecture/intro page with adr-14

[SebSept]

ADR-14 states :

• Core and PrestaShopBundle can now use Legacy components
• the Adapter namespace will phase out, not grow anymore.

This decisions should now appear on the documentation. I guess other pages need changes.

(It should be written with caution IMHO, so I won't make a PR)

[SebSept]

Since ADR is not accepted, this issue may deserves to be closed.

[matks]

This decisions should now appear on the documentation.

Good idea wdyt @eternoendless ?

[eternoendless]

ADR-14 has not been officially accepted but the majority of maintainers have verbally agreed. We need to

1. Formalize the acceptance
2. Update the docs

[eternoendless]

ADR-14 has been accepted, we may now update the docs