[website] use sticky table headers

The problem

When I visit https://microsoft.github.io/language-server-protocol/implementors/servers/ to check the implementation table, I often have to scroll back up to the top of the table to check the headers. If I'm checking an implementation low on the table, I'll see something like

| ... 
+------------+--------+-------+------------+
| a language | a name | a url | a language |
+------------+--------+-------+------------+
| ...

which hides which column represents what. Since there's no natural order to the list of features implemented, I'm unlikely to remember the column order.

Proposed solution

Use a position: sticky header on the <thead>s of the large tables.