libcurl documentation: updates HTML index

* Adds several links to documentation of library functions which were
  missing.
* Marks documentation of deprecated library functions "(deprecated)".
* Removes spurious .html suffixes.
lib/url.c: Generic read/write data pointers

Always interprets the pointer passed with the CURLOPT_WRITEDATA or
CURLOPT_READDATA options of curl_easy_setopt() as a void pointer in
order to avoid problems in environments where FILE and void pointers
have non-trivial conversion.
libcurl documentation: clarifications and typos

* Elaborates on default values of some curl_easy_setopt() options.
* Reminds the user to cast variadic arguments to curl_easy_setopt() to
  'void *' where curl internally interprets them as such.
* Clarifies the working of the CURLOPT_SEEKFUNCTION option for
  curl_easy_setopt().
* Fixes typo 'forth' → 'fourth'.
* Elaborates on CURL_SOCKET_TIMEOUT.
* Adds some missing periods.
* Notes that the return value of curl_version() must not be passed to
  free().
repository: ignore patch files generated by git

Ignores the patch files generated by the 'git format-patch' command.
Documentation: Typo in docs/CONTRIBUTE

Fixes a typo get → git in docs/CONTRIBUTE.