//index.h
const char MAINPAGE[] PROGMEM = R"====(
<!DOCTYPE HTML>
<html>
<head>
<title>Weather Observations</title>
<meta http-equiv="refresh" content="15">
</head>
<body>
<h2>Rain Gauge Two<br>Indianapolis, Indiana</h2>
<br> Last Update: %LASTUPDATE%
<br> Latitude : %GPSLAT%
<br> Longitude : %GPSLNG%
<br> Elevation %GPSALT% Feet.
<br> Temperature: %TEMP% C.
<br> Humidity: %HUM% %%
<br> Barometric Pressure: %PRESSURE% hpa.
<br> Rain Day : %RAINDAY% Day/mm
<br> Rain Hour: %RAINHOUR% Hour/mm
<br> Rain5min : %RAINFALL% Five min/mm
<br>
<h2>Weather Observations</h2>
<h3>%DTSTAMP%</h3>
<br>
<br>
<a href=http://%PUBLICIP%:%LISTENPORT/SdBrowse>File Browser</a>
<br>
<br>
<a href=http://%PUBLICIP:%LISTENPORT%/Graphs>Graphed Weather Observations</a>
<br>
<br>
<a href=http://%PUBLICIP%:%LISTENPORT%/README.TXT download>Server: README</a>
<br>
<br>
<a href=https://forum.arduino.cc/index.php?topic=606947.0>Project Discussion</a>
<br>
<br> Client IP: %CLIENTIP%
</body>
</html>
)====";
Now you have to write your own template processor, which returns right values for all %xxx% variables inside this template. If you need more help, then let me know...