Build requirements does not correspond to actual code status

### What happens?

1) In README.md, **Compile from source** paragraph there is broken link to [Build tools that are required to build DuckDB](https://duckdb.org/docs/dev/building/build_instructions).

2) They say in official documentation https://duckdb.org/docs/stable/dev/building/overview that C++11 standard is required. But pg_duckdb uses std::filesystem which requires at least C++17 compliant compiler.

Perhaps, it makes sense to figure that out.

### To Reproduce

Nothing to describe. Relates to documentation.

### OS:

Linux

### pg_duckdb Version (if built from source use commit hash):

0.3.0

### Postgres Version (if built from source use commit hash):

17.x

### Hardware:

_No response_

### Full Name:

Mikhail Litsarev

### Affiliation:

Postgres Professional

### What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have not tested with any build

### Did you include all relevant data sets for reproducing the issue?

No - Other reason (please specify in the issue body)

### Did you include all code required to reproduce the issue?

- [x] Yes, I have

### Did you include all relevant configuration (e.g., CPU architecture, Linux distribution) to reproduce the issue?

- [x] Yes, I have

Start with the Compile from source paragraph in README.md and verify the linked DuckDB build documentation. Then inspect the project’s build configuration and uses of std::filesystem to determine the actual minimum C++ standard. Done means the README link and documented compiler requirement accurately match the code and build setup.

Written by the indexing model from the issue text.