Documentation: add "Extensions to SQL" page

[ppom0]

Closes #877

I'm quite happy with this addition! I hope it'll save some time to newcomers.

However, I'm not convinced that the page belongs in 'Your first SQL website' directory.
Would you agree to move it up?

Documenting already took quite some time, and I don't have much energy to update this PR if you have suggestions.
So don't hesitate to rephrase and rework things if you don't like them!

[lovasoa]

Thank you very much !

I made a few suggestions, feel free to accept or reject them, then I'll merge :)

[ppom0]

Great additions, I learned new stuff, again :)

What do you think about moving the page to the website's root directory?

[lovasoa]

Yes, I'm ok with moving to the root!

[ppom0]

Done :D

[lovasoa]

Thank you very much!

[ppom0]

Just found out about the $id::integer syntax by reading this code.

Should it be added as well, or is it plain SQL? I don't know anymore 😅

[lovasoa]

The ::type syntax is specific to postgres: https://neon.tech/postgresql/postgresql-tutorial/postgresql-cast#postgresql-cast-operator

If you want to learn more about sqlpage internals, this ai-generated wiki is quite helpful: https://deepwiki.com/sqlpage/SQLPage

[ppom0]

Ok, I wondered because I saw this syntax in the following example, which is flagged as SQLite on the website.
Maybe its db engine category is a typo then?

https://github.com/sqlpage/SQLPage/blob/main/examples/corporate-conundrum/wait.sql

[lovasoa]

We have a special compatibility tweak, but it comes with a warning: https://github.com/sqlpage/SQLPage/blob/main/src/webserver/database/sql.rs#L910-L917

[lovasoa]

The sqlite examples that rely on it should be updated