Add filters documentation

docs/cloudtube/Filters.md

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+# Filters
+
+You can use filters to hide content on CloudTube that you do not wish to see.
+
+Content will be hidden from video recommendations, search, and subscriptions. It will not be hidden from channel pages.
+
+## By title
+
+Filtering by title is special because, unlike the other options, it supports pattern matching.
+
+Depending on the instance, regular expressions may be enabled, or they may not be.
+
+### Square matching
+
+Square matching is always available. Any text you type will be matched against the title directly, except for these special commands:
+
+- `--` placed at the start - allows other words in the title to come before the entered pattern
+- `--` placed at the end - allows other words in the title to come after the entered pattern
+- `[digits]` - matches a sequences of one or more digits, that is, characters in `0123456789`
+- `[choose|foo|bar|baz]` - matches exactly one of `foo`, `bar`, or `baz`
+
+Matching is case insensitive, so, capital letters do not matter at all.
+
+For example, the pattern `Minecraft[choose||,] but--` matches video titles that start with `Minecraft but` and `Minecraft, but`.
+
+### Regular expression matching
+
+Remember, regular expressions are not enabled on all instances.
+
+If they are enabled, put `/` at the start and end of the pattern, and the enclosed text will be treated as a JavaScript regular expression.
+
+## By channel name
+
+Select "channel name" from the dropdown and enter the full name into the "contents" field. Pattern matching is not supported.
+
+## By channel ID
+
+You may prefer to use channel ID over channel name if the same channel changes its name repeatedly.
+
+Select "channel ID" from the dropdown and enter the full channel ID into the "contents" field.
+
+You can also do this from the "x" menu when hovering a video thumbnail. Filters that were created in this way will also be labelled with the name that the channel had at the time that the filter was created.