Overhaul the usage of libcurl-impersonate (#4535)

libcurl-impersonate was not being used properly, as the code was
overriding the headers set by it to prevent detection.

- update the libcurl-impersonate to an actively managed lexiforest
  fork
- impersonate Chrome 131
- move the defaultHttpHeaders to http.php, where it belongs
- only set defaultHttpHeaders if curl-impersonate is not detected
- make useragent ini setting optional and disabled by default
- add necessary documentation updates

docs/03_For_Hosts/08_Custom_Configuration.md

@@ -119,7 +119,9 @@ Default network timeout.
 
 ### useragent
 
-Default user agent.
+Overrides the user agent value. Note that the default value, together with a set of other detection-preventing options is set
+automatically by the [libcurl-impersonate](https://github.com/lexiforest/curl-impersonate), which is used by the default Docker container distributed together with RSS-Bridge. Use only if you know what you're doing, otherwise you may stop libcurl-impersonate
+from doing its job impersonating real browser.
 
 ## Authentication
 

docs/09_Technical_recommendations/index.md

@@ -3,7 +3,7 @@
 ## Test a site before building a bridge
 
 Some sites make use of anti-bot mechanisms (e.g.: by using JavaScript) in which case they work fine in regular browsers,
-but not in the PHP environment.
+but not in the PHP environment. RSS-Bridge Docker container by default resorts to using libcurl-impersonate, which helps mitigating anti-bot mechanisms.
 
 To check if a site works with RSS-Bridge, create a new bridge using the 
 [template](../05_Bridge_API/02_BridgeAbstract.md#template)