Base: Add man pages for gettid(), getpid(), and getppid()

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/6b3a7b1ce72

Base/usr/share/man/man2/getpid.md

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+## Name
+
+getpid - get current process ID
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+pid_t getpid();
+```
+
+## Description
+
+Returns the PID (process ID) of the calling process.
+
+## Return value
+
+The process ID of the calling process.
+
+## Errors
+
+None.
+
+## See also
+
+* [`getppid`(2)](getppid.md)
+* [`gettid`(2)](gettid.md)

Base/usr/share/man/man2/getppid.md

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+## Name
+
+getppid - get current process's parent process ID
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+pid_t getppid();
+```
+
+## Description
+
+Returns the PID (process ID) of the parent of the calling process.
+
+## Return value
+
+The process ID of the parent of the calling process.
+
+## Errors
+
+None.
+
+## See also
+
+* [`getpid`(2)](getpid.md)
+* [`gettid`(2)](gettid.md)

Base/usr/share/man/man2/gettid.md

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+## Name
+
+gettid - get current thread ID
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+int gettid();
+```
+
+## Description
+
+Returns the TID (thread ID) of the calling thread. The first thread in a process has the same TID and PID. Subsequently spawned threads will have unique thread ID's, but all share the same PID.
+
+## Return value
+
+The thread ID of the calling thread.
+
+## Error
+
+None.
+
+## See also
+
+* [`getpid`(2)](getpid.md)
+* [`getppid`(2)](getppid.md)